Cut the frame pieces for the mirror. Cut the arc in the top frame piece. Assemble as shown with glue and 1-1/4” pocket hole screws. Place the screws so the router and rabbeting bit will avoid them.
Set the router bit depth to cut a ¼” rabbet. Cut the rabbet on the inner back of the frame to allow for the mirror. Use a chisel and hammer to square the corners. Adhere the mirror using mirror adhesive or a tab framing gun. The mirror frame can be attached tot he vanity with pocket screws through the side frame as well as metal mending plates.