We re-did 3 bathrooms a few years ago. All three had built in vanities that were non standard sizes. We ended up buying stand alone vanities that were slightly narrower than the open space. We then had custom counter tops made that extended to the walls on the back and sides. This is the brand/style we used.
https://ronbow.com/collections/all/products/60-torino-bathroom-vanity-cabinet
We were able to buy from an on-line dealer for much less than local. Contractor commented that they were of high quality.