Does it get wet from the shower or just from the sink?

I would explore having a complimentary tile on the vanity wall. Wallpaper may be messy if gets wet a lot. Is the shower glass a fixed piece on the left?

If it’s available, I would continue the black tile from the shower across the entire vanity wall and then keep the rest of the space light and bright.

Many small bath's are dark and moody...but they are usually powder rooms. If anyone is shaving at that sink I would improve the lighting. Either way, paint the wall white and look into a white granite with dark veining--opposite of the shower, as a wains coting that goes from floor to bottom edge of the mirror. Wrap it around to the left wall too.

You could have a mirror company make a simple piece that fits the back of the vanity from counter to underside of medicine cabinet. It could be caulked in with clear silicone. You’ll still have to wipe up drops but easy peasy on a mirror surface.

The shower is a glass enclosure. Gets wet from the sink.

I'm with Rachel- if you can match the shower do the whole wall with the granite

You could put a panel of glass like a splashback or acrylic wall panel that looks like glass. you would not worry about matching any colours.

https://www.theunderfloorheatingstore.com/lustrolite-over-basin-splashback?utm_source=google_shopping&m=configurable_scp

The nicest option would be to wallpaper that wall.

I would also do wallpaper and then mount a small piece of rectangular clear (not blue!) glass all behind the sink to protect the wall - it would be low key and modern

Got it! Much appreciated!