If your manufacturer doesn't make a filler, have them send you a door front and rip a filler from it. Black magic marker the ripped side before installation, please.

Finish the wall before you final install that or you won't be able to get it looking right. Is your counter going wall to wall, or just over the cabinet specifically? If it is going wall to wall, then I think you will need some small flat wood strip installed and stained to match the cabinet. Leaving that cap is going to create a space for stuff to fall into. I think you need a filler strip or something. I wonder if you could do a strip of the countertop material along there? Is it on both sides or just this side? With the light below, maybe an onyx strip molding along that side with your lighting below would make for a "finished" look. Another thought is if you are doing a stone countertop or quartz put a backsplash along that gap even if you have to double it. But do it on both sides so it looks intentional.

Can you slip a layer of sheetrock down that wall to the edge of the unit? That way the drawers can still slip by but it would be flush to wall? That might work better? Darn. I just isn't going to look right with any trim I don't think. I am still thinking! bbl

Yes create a matching filler

Thanks to all for your replies - Flo, the other end is open and the toilet goes next to the shower(former tub shower)- The countertop will be Hanstone Quartz- English Toffee and will go all the way to the wall and It wall have a 4" splash. This was installed and then moved already. If anything falls in the 1 inch crack next to the wall (if the top covers the opening it seems unlikely) it will land on the floor. The cabinet is 10" above the floor.-I called the manufacturer, Wyndham Collection, and they do not have filler pieces. Not sure how easily I will be able to match the color on a contractor made filler piece.--Maybe the drywall will work but do I try to match the cabinet or go with the wall color when I choose it? If the vanity went to the floor with a traditional toe kick I would certainly agree I need to fill the space, but as this is floating do I really have to follow the same rules?

Filler it is- Thanks!

Matching filler is probably best best given the details you provided. With the Quartz going to the wall that helps a lot. I wonder about doing the backsplash then? If the Quartz is measured and installed perfectly I would leave that backsplash off. Kind of tales away the modern look. What hardware are you using on the drawers? Great drawer pulls can distract from other details as well as cool lighting. Seems like cabinet maker could make a strip of trim so you match well and make things look right. But is that isn't an option take a drawer to a good local cabinet maker and get them to match and do a flat trim filler piece. Good luck.

Thanks again Flo! Per your backsplash question, this is a kid's bath remodel (that I should have done 7 years ago) so I limited myself to remnants to keep costs down. I could not find a tile that I liked to use as a backsplash, so I feel like it's the best way to go.- (no splash always bothers me and my boys are messy). I am thinking about running the new mirror to the ceiling and putting the new lighting fixtures on top, so that might help keep it modern?Thanks again- just got back from Sherwin Williams with a paint match :)

"I called the manufacturer, Wyndham Collection, and they do not have filler pieces. Not sure how easily I will be able to match the color on a contractor made filler piece.-"

This is why you buy a matching door and rip it.

Cut out a portion from the side that will be hidden and use it as the filler...? Then fill up that hole from the inside with a piece of laminate.

Joseph- I will try to order a door with Wyndham Collection, if my match is not close enough. I am concerned that the finish will splinter when you cut it. Fortunately this is in a inconspicuous corner and the paint match should be ok- we shall see. If I could not see the wire running to the light under the cabinet, and I had finished and painted the wall before cabinet was hung, I think everything would look ok without filler. Oh well you live you learn.-- Thanks again everyone for all of your suggestions.

Use the right saw blade and and they'll be no chipping. Run a little color matched magic marker on the ripped edge and it will disappear.

If you use a simple piece of wood as the filler set back approx 1/2 from face painted wall color it will keep the floating free piece of furniture look. You have to decide the look you want but for a floating vanity it looks better to me not attached straight o wall.

When I build a piece that looks like furniture but is in a built in situation i use the above.