Unscrew the handles and knobs from the vanity with a screwdriver. Mix 1/3 cup trisodium phosphate with 1 gallon water in a bucket. Submerge a sponge in the mixture and wipe down the vanity, scrubbing it thoroughly. Wipe it down with old towels.
Press a piece of 100-grit sandpaper onto the bottom of an electric sander. Run it all over the surface area of the vanity. Repeat with 220-grit sandpaper and then 360-grit sandpaper. Wipe off the dust from sanding with a tack cloth.
Cover up all areas you don't want to stain with newspaper and painter's tape. For example, you'll need to cover up the mirror section of the vanity in this manner.
Squeeze out a generous amount of stain onto a foam brush and coat the wood vanity, making smooth, even strokes. Allow it to soak for 15 to 20 minutes and then wipe off the excess. You want to choose a stain that accentuates the grain of the wood. Repeat once or twice, depending on how dark you want the vanity to be.
Using a 3-inch brush, coat the vanity with a light coat of clear varnish. Allow it to dry for 24 hours. Polish the handles or knobs with a soft cloth and metal polish or replace them with brand new fixtures.