And your question is???

Replace the entire vanity or just replace the top sink with a new countertop and vessel sink? Cheaper for one or the other, or about the same cost. Replacing the entire vanity will have to find something that fits the existing footprint.

Seems to me that with new counter & vessel sink the only thing significantly raised is the bottom of the sink. You’ll also have to do some plumbing modifications to add under sink piping & of course now there’s a problem with either where backsplash was with the old counter or with the mirror which starts at backsplash height. Of course you’ll also need new faucets. It’ll be like pulling a thread. Do you like vessel sinks? They‘re decorative & generally a lot more costly than a bathroom sink which you can get for less than $100. If you already know it’s hard to find a 40” counter (I don’t know that), then how about get a new counter, you can get a quartz or granite remnant from a stone fabricator, get an undermount sink to maximize counter space, and have the vanity built up from the bottom. I know that can be done - a frame built to raise the vanity. Check out having vanity professionally refinished to clean up the oak - or professionally painted if you want to change color. Can’t say how much this will cost but how long will you be in this house & how much inconvenience is the too low vanity for the people using it every day? That will tell you if it’s worth it. It’s also a value add improvement to house.

A 31" vanity is short. Standard these days appears to be 36".

I wouldn't use a vessel sink in a bathroom. Too hard to clean; you can't push gunk that accumulates on the countertop into the sink.

The floor tile may extend underneath the vanity. Without removing the vanity, you might remove the toekick or cut a hole in the cabinet bottom to take a look.

There are 40" wide free-standing vanities available. For example, the Fresca Tuscany and Fresca Valencia.

I had to move some plumbing in the wall to accommodate a new vanity that has drawers from top to bottom.

Hanging the vanity on the wall can make the room look bigger and more modern.

Absolutely buy a new vanity. There are so many to choose from in so many price ranges.

I know someone who had matching wood base put under existing vanity to raise it to the desired height.

I faced a similar dilemma when renovating my powder room which also has a 31" tall vanity. I choose to go with a vessel sink and keep the existing vanity because of its odd shape. I like the result, and the height does help since you do not have to bend down to wash hands. Although this Kohler sink was not inexpensive, there have been no problems with cleaning or splashing. I believe the splashing can be minimized with a combination of faucet placement, spout height and reach. The other option I was considering is building up the top of the vanity with a wood frame, which would have been hidden behind taller replacement doors. Not sure you have that option since you have drawers at the top of your vanity. You also need to decide whether you are keeping the mirror as this will likely limit the height of a new vanity. Good luck with your project!

I have a very useful vanity from Fresca with two drawers at the bottom and two doors above. Bottom drawers are a great design since they are below the plumbing, and cleaning supplies work very well between the plumbing and are high enough to reach easily.

BUT... I got a Fresca vanity in cherry wood stain. There are a few places on the body and on the doors where wood was pieced together with that zig-zag piecing method. These are smooth enough that they cannot be felt, nor does the texture change at all to indicate that these are there. But since I got a stained cabinet instead of a painted one, the pieced-together spots are visible through the stain. So I advise to get only painted vanities from this company. My contractor did not notice this until I pointed it out, and then he was all “what the...?” Like I was! Not worth sending it back, though and it is the only thing wrong with the order. Good quality and solid build, otherwise.

@bunnybluebird Did you also get a new countertop?

Yes, new countertop, sink and faucet. I did not end up needing any extensive plumbing work, but your set up may be different. This is what it looked like before (laminate counter).