You must leveling a bathroom vanity top before attaching it to the wall. If the floor or wall is not level you will have caps to close with your molding. Below are the steps you should follow in order to create a level vanity top.
Step 1 - Move the Vanity
Move your vanity as close to the attaching wall as possible. Do not pull it out of place.
Step 2 - Screw and Adjust
Use shims to level everything. Attach it by screwing it together while using the shims and the level to ensure that the vanity top is in proper position.
Step 3 - Finishing Up
Use screw gun and screws and attach it all together. If you have followed the above steps, you shouldn't have a problem with your vanity being uneven at any point.