I have removed vinyl wallpapers several times before, and in my case they had a paper backer below the vinyl. The vinyl peeled right off like the skin off a banana, leaving the paper layer stuck to the wall. I soaked the paper with lukewarm water mixed with dishwasher soap in a hand sprayer, and kept it saturated for about 5-10 minutes. Then I could scrape it off with minimal effort with a putty knife. After that I washed the leftover paste thoroughly with 1/4 cup Mr. Clean in a gallon of warm water twice, then I rinsed thoroughly.

Google mural seascape wallpaper. I got one similar https://wallpapersafari.com/w/jfiFEh

I love the vanity so please do not ruin it by painting. I think maybe just a big piece of art on the large wall and walls the same color as the white in the floor.IMO any patterened wall paper will fight with that floor so at best a textured wall paper maybe. I would paint one wall black and put that art on that wall

A mirror can be busier than a painting. Depends on what the mirror reflects, and what the painting is. Avoid dining room mirrors if they reflect the diners as they are eating.