I have some ugly tile in my bathroom and want to put down a piece of vinyl instead. The tile was installed over concrete and the toilet ring is short already so just wondering what to put over the tile to prepare it for vinyl.

Is the tile now.. smooth, attached and in good condition? If it is then I'd simply clean it with a good cleaner that will strip wax that might inhibit the adheasive from bonding. If there are chunks missing, loose tile or other major defects then there are other steps to take.

I would skim coat the existing floor using floor leveler, prime the floor and then put the vinyl down. This way, you'll have a smooth sound base for the new adhesive.