A lot depends on the shape of the integrated countertop back and side edges (curved or level). Do you have a link or photos?

of course, here is a photo of what it looks like on a random base (my base won't have legs)

. it's level.

If you used a local corian fabricator you could have an integrated sink and counter custom made for the space.

This is fraught with issues.

If you use a vanity with legs you don't use fillers. It's just not done.

You can't center the vanity and then leave space on either side because you can never paint that area.

You can use a 49" vanity and use fillers but get a top cut to fit the alcove and do a correct installation with a backsplash. BTW can you even order matching fillers?

Have you thought of simply ordering a vanity from Lowes or HD through the Kitchen Planners Dept? Any kitchen cabinet manufacturers repped by the store have vanities that can be ordered from their catalogs.

thanks! i am not planning on using a vanity with legs. the base itself will be perfectly flush to the wall. the issue is the countertop. my budget does not allow for a slab to be fabricated hence the integrated sink approach. what i am trying to understand is how best to close the gap between the vanity top and the wall.

What about a cultured marble top made to fit?

Why are you remodeling? Your vanity looks great. You could just epoxy the old top to save $$$

Ditch it all, tear it all out mirror as well and float the vanity. As for the backsplash just run a entire slab of stone of your choice up that entire back wall the mirror is on. But definitely ditch the vanity..no point in having side filler space you cant use.

is it mandatory to do an integrated sink? if this is what you really want, then you're going to have to get a custom made vanity and top to fit.

otherwise, you'll have to compromise. Here are some ideas.

can you find one in a custom size to take it wall to wall?

(you could just keep your current vanity, have it refaced and painted, and add a new top)

If you do this, you could get a custom made top w/your sink to fit on top of the old vanity, then just get new fronts and paint

https://southernhospitalityblog.com/how-to-update-an-old-vanity-with-new-drawers-doors-and-paint/

Or do the 48" w/a filler

these pics show the fillers

This one appears to be custom built for the space. I'd try and do this

another custom:

Or, try something like these

an inch or two on either side isn't a big deal

floating vanity w/wall mount faucet is always a good idea too

Rather than worry about shimming out the vanity, have a drywall person come in and add drywall to side walls so the space is the correct width.

thanks for the comments, everyone!

I actually did a vanity, almost identical to what you are considering. I used a premade 49 inch freestanding unit that I bought online from Costco.
It does have a backsplash, although no side splashes, which I think are really old looking anyway. Also, it just seems to float in the room giving it a very nice “furniture“ like effect.

I tried ones from Amazon, but I’m always the best quality, surprisingly Costco has great stuff

thanks! my concern is really the countertop. the vanity base itself will be flush because we will do fillers. that how the current base is fitted. but a 1.5 inch gap for the top on either side will make it difficult for cleaning and potentially might contribute to mold. i can't afford anything custom made. i thought about fillers for the countertop as well but fear that it may not be aesthetically pleasing. the only other thing i can think of is doing trims to make it watertight.

to answer some questions:

why am i changing vanity and top? simple: i don't like it. it may work for others but it's not vibe.

why not built something custom? not in my budget unless you willing to fund it :-).

why not do a floating vanity? because i would have to finish the floors and can't afford that.

why do an integrated sink? it's the most cost effective. i don't have to buy two separate things and fabricate a slab of something.

Sure, that makes a lot of sense. If it were my bathroom I would keep the cabinet and perhaps refinish it. Also, the best money spent is on knobs and handles which can be easily found. Then you could get a refinishing company to put a coating on the top, which looks just OK but should last a couple of years. Another alternative is just having a marble/granite fabricator cut you an exact piece. They could probably make it from some extra pieces that they always have, and just bolt the existing sink to it.

super helpful, thx!

You’ve said your current vanity just isn’t your style and the best option is to remove and replace…which is totally reasonable. But don’t let compromise become your style especially in a room that is such a workhorse as a bathroom. Keep saving money until you can find a countertop that checks your style boxes without needing strange compromises in order to make it work.