Hi Arnie,
I did a lot of editing with a freeware tool named "audacity", which is actually a very powerful software.
But your plan sounds difficult to me, or impossible, because you want to take vocals from songs and want to place them over other songs.
So you have the problem, that you would have to extract the vocals, cause otherwise you would just mix two songs into each other.
I don't know if there is any software which can do, what you want. :-/
regards
Sometimer