Fabulous question--and it sounds like a fabulous project. Repurposing a piece of furniture to function as a bathroom vanity is a great way to make the space uniquely yours while saving money. I did a similar project where I repurposed wall-mounted storage chest from IKEA to serve as a wall-mounted vanity. Much like you, I wanted to protect the MDF from any water splashes that are bound to happen during hand washing. I sought out a local glass cutter who was able to cut a piece of glass to fit the vanity top and then cut a rough hole in the center to accommodate the plumbing. What kind of sink do you plan to use? I learned the hard way that class cutters cannot cut a perfect circle in glass (so much for the spy movies!). If you opt for a glass top, make sure the sink will hide the roughly cut hole. It might be worth it to have a professional assist you with this project, at least in the beginning. I'd hate for the sink to be too heavy for your dresser's top. If extra support on the underside is needed, they should be able to help you out. Best of luck with your project, and please post pictures once it's through--we'd love to see it!