So are you saying that you'll have 9" of empty wall space on one wall? If so, I'd try to find a 36" vanity so you don't have that dead space. If you can't, I'd butt the vanity against a wall, and extend the countertop the full 36" [have to build a support around the dead space to hold it] so you can use the other 9" for storage--maybe a narrow clothes hamper.

I wasn't sure that would be a good idea because then the cabinet would be right up against the bathtub for 21 inches. But I sure need storage so if you think it would look okay, that is a good option. Thanks!

You need to also check your city or county building codes regarding how much room you need between the toilet and tub and vanity before committing to anything.

The issue with the vanity comes down to what looks the best given the situation you're in.