I have a small bathroom with self-adhesive vinyl tile on the floor. Can I put ceramic tile over this vinyl tile?

I'd take them off. It won't take too much time.

A propane torch will work well to heat up the tiles. Use a 2" putty knife or 4" drywall taping blade to scrape them off while you heat them.

I'd probably remove 'em too, but you could glue and screw 1/4" hardibacker down and tile over it (pretty easy if it's wood subfloor).

OK we all agree, that up the old self adhesive tile before it takes itself up. Like Art I am a huge fan of Mr. hardie's products like Hardibacker and Hardiboard. Be sure to use the special green screws they sell for Hardibacker, 1 1/4 ' screws for 1/4" boards and 1 5/8 for 1 1/2 " boards. Mud the seems with the same modified thinset you will use to set your tile and use fiberglass tape, I never do but Mr. Hardie likes it.