You can try to put subway tiles there rather than a one piece vanity backsplash. The waves in the wall should be easier to disguise with smaller backsplash tiles.

Will there be a sidesplash to cover the gap there? If it's just the back wall you're concerned about, a good drywaller should be able to use drywall spackle to feather it out so it's filled in better. Likely use a caulk after to finish the edge at the wall.

Your contractor should KNOW how to do this!

You need joint compound, a level......and a tutorial. along with a side splash: ) and finally, caulk. Easy peasy, and some patience

https://www.youtube.com/watch?v=swJoX7br1U4

As Jan mentioned above this should not even be needed to be thought about nby you. Your GC needs to have the drywall guy or the painter "float" the entire are of the wall, they fill in the low spots with joint compound feathered out smooth and then the wall gets primed and painted again. The fact that you contractor even needs you to think about it is a little unnerving in my mind.

Terri, you need an experienced contractor who knows what he is doing if you want to make it look nice and clean without going through lots of trouble.

You always expect a certain amount of tolerance in framing, etc., and there are several methods experienced contractors use to handle most situations and get a nice outcome.

That being said, to start, it looks like your countertop is too far away from the wall. It should be right against it. Is the vanity installed tightly to the wall, or does it also have a gap in it?

Everything needs to be assessed, starting with the vanity installation; it must be installed level and plumb. If something is off, it should be corrected at that point. From there, everything else will go as it should.

Good luck.

Will suggest you have a contractor come and look at your wall to determine if there's anything else going on besides just irregular framing. Are you having any trouble with cracks elsewhere. Might be due to a settling foundation.