I'll be curious what the pros say, but I would imagine it to be very difficult, if not impossible, to get it perfectly level. In fact, it's likely your tiles are not all level and "in plane" anyway, having high corners every once in a while. (But maybe I'm wrong.)

Real good answer. Sheet vinyl or any other relatively thin and resilient flooring needs a smooth and flat substrate. If the existing tile job is flat with very little lippage, then preparing the surface to receive the new flooring would be easy and cost effective.

As others have said, you must get a smooth and flat substrate or "slight variations in the surface will show through"...i.e. 'telegraph' to the surface of the vinyl.