Ideally you should lift vinyl or apply a new subfloor. That said – two yrs ago i was in the same situation, and couldn’t remove the my sheet vinyl from the concrete slab, so i laid the new tile over top. You must pay particular attention to the type of adhesive you use. Make sure it is for this application. It costs twice as much as regular thin set, but it’s a cost you can’t ignore if you are going to do the job right.