Tiling when some old vinyl sheet flooring is impossible to remove
I would rent a power chipper, these have a blade that vibrates back and forth that chips up tile or vinyl on concrete floors. Since a bathroom is small a hand held oscillating multi tool could also do the job and you would have a handy tool for other jobs. I do not like tile over vinyl and the places I have found it I was usually pulling the floor up because of problems with tile or grout cracking.