We used a single row of neutral tiles across our vanity as a backsplash. Our mirror fit up next to them. Worked well and was in the kid’s bathroom so lots of use.

My vanity was installed with a large mirror behind it and they caulked where the edges meet. Not sure what you are planning for the mirror/sconces but perhaps something like the below would work?

This is my vanity counter during & after my install.

Here are some examples for you.

The price of some simple tile for a backsplash will probably not cost much more than repainting the wall forever.

Vanity as furniture piece is a good look, but I’d do a tiled backsplash here. Stone is going to be too thick and it’s very unlikely you’ll get a match at that size anyway. Tile the wall behind up to 42” or so. It’ll look nice and you won’t have to worry about painting that entire wall every time it starts to look bad.

And I wouldn’t put mirror or glass behind a faucet (near toothpaste and soap) if my life depended on it. Your bathroom is so pretty! A plain tile backsplash will keep it that way ;-)

I’ve had no backsplash & just a mirror for over a year and have had no issues. The mirror does not get wet or splashed but it’s completely caulked so if it does, just wipe it up. :)