this is always a would you wont you topic,in my opinion if the client wants it the client gets it,point in question i was working on a job last year doing communal areas in tenanted flats on the landings were vinyl tiles i insisted that they be taken up,the client did not want this and bypassed me and went to the supplier(a very well known spanish company)there rep said that the adhesive was suitable to go over the vinyl tiles as long as they were sound.
Dont now about you guys but i would rather go over ceramic tiles than vinyl tiles,by the way that practices has gone on now for well oover a year on 100s of these blocks i dread to think the consequences of the reps decision.
Too the original topic if the floor is sound tile over it ,BUT make sure your written quote states that you give no guarantee if the substrate fails.