We removed the old vanity lights & the walls are just plaster
what shall we apply on walls to attach the new lights?:confused:
Depends on the fixture, some require a box to attach to, some attach to the wall. For plaster, predrill a hole and use a hollow wall anchor, there are many types to choose from.
If the replacement vanity light weighs less than 6 lbs. you use what is called an "old work" round box.
Leave about a foot of wire hanging out of the wall in the new sheetrock, cut a round hole where you want the light, insert the box, tighten three screws in the box and mount the light!