What kind of tile are you installing ? Vinyl over vinyl is done all the time as long as the existing vinyl is not 12' squares, and is not a perimeter bodn i.e. full spread glue underneith.
With a sound, full spread, sheet- vinyl floor, you have the options of not taking it up, and taking it up. If you choose not to, then emboss the floor, wait till it completely dries, sand the high spots, vacume, and your ready for the new vinyl.