First of all...don’t panic! Get some dark brown paint, closely matching the color of your vanity. What material is your cabinet made from? You might be able to get away with using paint from a hobby store. Get a slightly darker color, too. Then paint the part that is exposed with the paint that most closely matches your vanity. It will still be visible. The trick will be to lightly feather in a darker color so that it appears to mimic the “wood” finish that was on your vanity. You could also try Minwax stain pens, sold at hardware stores. I have had a lot of luck with those on scratched furniture and floors. Where is this located in your bathroom? It might be something that only you will see. Good luck!