Waterproofing between wall and new sink vanity
Yep, PVC trim, Accent-strip Tile, Slices of a 12x12 (whatever size you desire) Tile or a sliver of Granite would probably be the best and not be problematic with future cleanings removing a finish or paint.

Or, you may want to reverse your thinking and frame around the entire mirror with a decorative PVC molding. This assumes the mirror is gapped all of the way around for the best look.