How easy or hard is it to remove a vanity w/sink and replace it? Is it just a matter of turning off water and unhooked pipes?
If it is the same size yah turn off water and disconnect plumbing, reomve sink,remove top, remove vanity install new vanity, install counter top install sink hook up plumbing and your good to go. You may face a few details while doing this and alot has to do with what your putting in there.
1- Is the vanity cabinet all one piece?
2-Is there a new sink and taps or the same ones( you will need new supply lines no mater what) or is it one of those tops that the sink is molded into the top
3- Got a picture of the old one and the new one?
What about putting in a recessed/inserted type medicine cabinet? Just make the hole and put it in, screw in place?
This realy depends on the size, if it is sized to fit in between two studs ( 14-1/2"" wide then yah cut the whole and install but again do you have a picture of it? It is possible you might find a wire or plumbing in there and you will have to deal with that if it is the case. If the medicine cabinet is going directly over the sink( centered) you could very well find the vent pipe so do some investigating before cutting into the wall.