How can you turn antique vanity dresser with mirror into a desk?

Take out the drawers and turn into shelfs

remove the mirror and add a shallow hutch to provide additional storage space.

Remove the mirror , keep the drawers as you'll be needing them , but remove the top two when you use wood screws attach the 3/4 piece of plywood you have cut to make the surface of the desk. It should be stained to match the rest of the piece and you should apply several coats of polyurethane (sanding lightly in between) before attaching it to the vanity that is now your new desk. Take pictures and post to Hometalk.com so we can all ooh and ahh at your new desk!