Bathroom vanities may be plumbed in 2 ways: The plumbing for the water and waste might be found from the wall behind the vanity and come through the back, or it might be found from the ground and come through the base of the cabinet. While it may seem more difficult to get rid of a vanity with the plumbing coming up in the underside, it actually detaches quite similarly to a vanity plumbed from the back. Both need detaching the pipes that hold the tap and sink set up.
Shut off the water to the bathroom. Depending on the magnitude of the holes cut at the base of the old vanity, you might have to take the valves away from the pipes to remove the vanity. Do not rely on shutting off the water beneath the sink, since this could still lead to water flowing up through the ground.
Disconnect the “U”-shaped part of the waste pipe where it spreads to the sink. Use a basin wrench to turn the connection counterclockwise until it comes loose. Place a bucket or bowl beneath the pipe and then disconnect the end of the “U” that leads down to the base of the vanity. This bend is full of water; drain it to the bucket as you take it outside.
Disconnect the water supply lines that lead to the tap in the pipes coming up through the base of the vanity. Loosen the nuts that hold them in position by means of a basin wrench and twist them until they are free.
Examine the base of the vanity. Have a look at the magnitude of the holes cut at the vanity to adapt the waste pipe and the water supply lines. If the holes are big enough to accommodate the pipes and the valves, then no action is needed. If the holes are too small to accommodate the valves, then use the basin wrench to detach the valves. Place a plumbing cap above each of the supply lines to keep debris out of the pipes as you work.
Detach the vanity top in the walls about it by running a utility knife around the perimeter to decrease the sealer holding it in place. Detach the vanity top in the vanity itself by running the knife in the link between the top and the cabinet, until the shirt is free. Lift it in the vanity.
Look carefully at the back of the vanity where it meets the wall. It might have been screwed to the wall at the top back, beneath the vanity top. Unscrew the vanity in the wall and get an assistant to help you lift the vanity straight up until it’s off the pipes, then transfer it away from the wall.