More information would be helpful. Questions:
-Is this range in an island or against a wall? From the VAH website it appears the Excaliber is available as an island hood or an undercabinet hood.
-What size hood is your appliance store recommending? It can't be a 44 inch hood--the VAH hoods appear to come in the usual 42 and 48 inch sizes, like virtually every other hood out there. Upsizing to at least 48 inches would be customary for adequate performance.
-Why would the Excaliber hood be considered "custom"? It's an off-the-shelf model.
-Not sure what you mean by "vents up not out." If it is an island hood, all island hoods have to vent vertically, at least up into the ceiling. Wall mount hoods can have an option to back-vent horizontally directly into the wall and VAH makes models that can do so. You need to figure out how you are routing the ductwork--i.e. is the range on an exterior wall?-- and then do some homework. Check the VAH website for specs on what they offer. Look at some of the well-regarded alternatives--Modern Aire, Prestige, Independent, Wolf--and you should be able to get some idea of what you want. And search this forum for tons of good discussion on ventilation.
-Consider getting a new appliance store. They sound clueless, or desperate to unload a VAH Excaliber on you. And be suspicious of contractors who offer to "make it work." He should be able to explain the ins and outs of ducting a hood, and if he can't, then find someone who can!

Thanks clingresga for your reply. Let me answer your questions.
1. the range is against an exterior wall
2. I would like an undercabinet hood
3. I ordered a 45" hood, hence the custom size, based on the information we had from the appliance place.
4. My understanding is that Excaliber used to be able to vent horizontally and vertically, but now it is made to vent vertically only.
I will check the websites of those you mentioned. Thanks.

1) Do you want to vent straight out the exterior wall, or run up into the ceiling? If into the ceiling, where will you go? Horizontally out the same wall, only higher, or all the way up to the roof?
2) understood
3) There is no reason the hood size needs to match the range size. Indeed for optimal ventilation it really should be wider than the range. That would also avoid the need to order a custom hood--a 48" hood would have been optimal.
4) Given all that, I would argue that they didn't do a good job of advising you. Might it be possible to claim that they misrepresented the Excaliber's ability to vent horizontally, and thus you are not responsible for taking it? Otherwise, as a custom unit I'm assuming you're stuck with it
5) do take the time to search ventilation and hoods on this site: it's a great resource.

Thanks for hanging in there with me clinresga!
1. I want to vent straight out the back through the exterior wall. Venting vertically seems problematic, I have the cabinets installed already, there is the master bath above the range, etc. Would it be weird to vent vertically up to the roof? Would you see it in the bathroom or would it be behind the drywall?
2. We were advised to get the 45" vent and now if we tried to do a 48" we would have to take the new cabinets down and reconfigure them, I think, to leave enough room
3. We are not stuck yet with the vent, it sounds like not much has been done at ventahood in the month since we ordered it.
4. I will check this site out more, thanks.

1) Venting through the wall should be easy, as long as your hood allows back venting. The VAH site goes into details. Most VAH hoods do not directly vent back, but all can be used that way by using a 90 degree elbow emerging from the top of the hood which would "turn" the ducting horizontally. It will of course protrude out on top of the hood. The SLH6-K 6" undercabinet looks like it could vent straight back without a visible elbow. None are listed as available in custom sizes, although that may still be a possibility not mentioned on the website. Doesn't sound like it's worth going vertically as you'd end up having to build a chase through the bathroom etc.
2. Still confused about the 45" recommendation as it's custom size for every company I know about. Still, sounds like you're stuck. May be a challenge to find a custom undercabinet hood, as most custom hoods tend to be the larger freestanding models, or liners intended to go into a custom enclosure.
3. I'd look at alternatives, some of the companies I've mentioned. I've had great responses from the folks at Modern Aire (www.modernaire.com) who could give you an idea of what product would work and what it would cost.

All in all, it won't be a "state of the art" ventilation system, esp given that you have such a beautiful range!--but it should suffice given your constraints.

you may want to do a search for metallo arts, they use ventahood and know the product line well

I think Metallo Arts may present the same problem: they list 42 and 48 inch widths, like everyone else, while kkim sounds locked into 45 inches. If that's really true, then a true custom company like Modern Aire may be the only option.

Kkim: how sure are you about the 45 inch width? I still don't think VAH makes that size and am confused about what you had ordered previously. If it turns out it was 48 inches then you have many options.

ACTUALLY METALLO DOES ALL SIZES AND SO DO ALOT OF OTHER COMPANIES BEYOND METALLO AND MODERN AIRE...... BUT WHAT DO I KNOW

Sorry, didn't meant to get you shouting! : )

My apologies if I was in error. I based my reply on the Metallo website. I cannot find any mention there of custom width hoods. If you use their "Rangehood Builder" app, step two is to choose range hood height and width. The width options listed are the usual: 36'', 42'', 48'', 54'', etc. No mention of option to customize width. However, realize that I did not go as far as to contact them myself, which you may have done. They may indeed have capabilities they choose not to list on their website.

I am sure that they could build a custom hood ENCLOSURE to 45'' width--but I had assumed that kkim was looking for a hood, not a liner and enclosure, based on her initial order of a VAH Excaliber undercab unit. The problem with a custom enclosure would be that if she used an off the shelf hood liner it would be a 42'' liner, and that's even more undersized for a 45'' Aga range. Best option as you know would be to find a way to upsize to at least 48''.

And yes, thanks for the reminder, as I had stated in earlier posts there are many companies that will custom build hoods and liners besides just Modern Aire--Independent, Prestige, and others that have been mentioned in other threads here.

Kkim...I suggest you measure the actual space b/w your cabinets. Sometimes KDs leave a few inches b/w the cabinets and the hoods...w/o actually mentioning it.

Or KD originally left approx 5" b/w hood & cabinet on each side. (We're down to 1" now, though, b/c of the 7-1/2" measuring error on that wall that ate up most of that space.)

As others have mentioned, ideally the hood should be approx 6" wider (or more) than the cooktop/range for better capture area. E.g., we have a 36" cooktop and our Vent-A-Hood is 42"

If only I had consulted with you beforehand! Yes, I am locked into the 45" size by the cabinets, but also by the limits of the wall, and the centering on the window on the left side. I have since seen in many places, not only on the site, that we should have just gone with the 48" vent. Hindsight is 20/20.
Another wrinkle is the height of the vent. We ordered a 9" x 45" vent, which we found does not vent horizontally, presumably because the venting ductwork is 10" (or so says the appliance guy). Sooo.....
1. I could order an 18" x 45" vent which would vent horizontally. Then I would have an 8" gap at the top of the cabinet run that I would have to figure out what to do with.
2. Modern aire makes a 10" vent that can be made a custom size but can only handle 400 cfms and I need 600. I could also order an 18" x 45" vent from them.
3. I could stay with the current size and make a elbow in the ductwork to go vertically then horizontally.
4. I can keep looking for other options...

Which are-
Metallo, prestige, independent and what else?

kkim: well, that explains some of your issues. Some questions and thoughts:

I am assuming that all of the dimensions you refer to (9'', 18'', 10'') refer to the HEIGHT of the hood.

What did you plan to put above the original hood? A cabinet?
If so, what about using an elbow to go vertical to horizontal, hidden inside the (unfortunately not very functional) cabinet? At least it would look OK and allows you to go with the 9'' vent.

If there is nothing above the hood, I guess you could have someone fab a "fake" chimney running vertically to the ceiling, which would conceal the elbow, and look like it was a vertically venting hood.

Or, if there is nothing above the hood, what about a custom hood enclosure that goes up high enough to conceal your elbow? Though this will likely force you to go with an even narrower hood liner, probably not a great idea).

If you're going custom, you could consider a deeper than normal hood to make up to some degree for the narrower than optimal 45'' width. Need to watch out for taller folks who might hit their head on it though.

If you have a layout you could post it would be helpful.

You list many of the good custom hood companies. Search this forum for other suggestions.

Hang in there! It will work out fine no matter what--98% of the time the 45'' hood will be more than adequate, and 2% of the time when you sear or wok something you may get a bit of smoke spillover, certainly not the end of the world. Just make sure it's as pretty as the Aga.

thanks, clinresga...what does that mean? just curious?

Yes, the height of the hood is what I am talking about, the width is 45".
Yes, I have cabinets installed about already, this kitchen project has been going on since last summer. The original appliance store we ordered from went bankrupt and we didn't find out for 4 months, I was calling weekly for status reports and getting nowhere, of course, they never ordered it. We ordered from appliance store #2 and found out 7 weeks into waiting that we had this issue.
You have some great suggestions, I will talk to the installer about them. I didn't think about faking a vertical chimney, that might look good. I don't think I will go with a vertical vent, since it just makes everything more complicated. Confused about what a hood liner is. and a hood enclosure. Thanks, "c"

another thing you could consider doing is bumping out the cabinet that's going above the hood. if it's a 12" depth cabinet, like most uppers, you could bump it out 10", to bring it out almost even with the front of the hood. this would create a 10" opening behind the upper cabinet that would allow you to run ducting back there.

that would make your cabinet functional and would also make it aesthetically nicer since the hood won't appear to be sticking out so far.

kkim:
clinresga is an acronum from a running family joke. No great signficance.
I'm still not totally clear as to whether there is a cabinet ABOVE the hood (in other words, the vent you have ordered would be a true "undercabinet" hood that is attached to the underside of the cabinet above it). If that's the case, I like edlakin's idea of bumping out the cabinet above the hood, giving you room to hide the elbow, and making it look more cohesive.

If there is not a cabinet above the hood you're getting, then the vertical fake chimney idea, or building a hood enclosure to hide the hood would be options, although if you are definitely getting a hood, not a hood liner, then the latter doesnt work as easily.

A hood liner is a simple hood fabricated out of sheet metal. It is functional but not intended to be seen, as it's "unfinished" on the outside with exposed seams etc. It is placed inside a hood enclosure, which is something custom built, generally by the cabinet maker or a company like Metallo Arts. It can be made out of almost anything--wood, metals like copper, stone, etc. It allows you to get exactly the look you want rather than having to go with an off the shelf hood. It's popular as folks go with larger vents and more expensive redos.