Adhesive for tiling entryway w/porcelain tile?

I,am about to start a tiling project using porcelain tile and was wondering if there is a special thinset I need to use in order to tile porcelain. I currently have a vinyl sheet flooring which has plywood underneath and was wondering if i could apply the thinset directly over the vinyl floor or if its better to apply it to the plywood itself in order to attach backerboard or do I even need to apply backboard and should I just tile over the vinyl flooring or plywood. I would like to use a premixed thinset if possible. Any suggestions would be helpful.