I have a tongue and groove flooring, on top a 1/4 inch sub flooring, vinyl flooring on top of that, can I use hardie back without using the thinset? Can I just screw it into the vinyl without using the thinset?

All cement boards need thinset under them on all floors and countertops and tub decks. All of them. Pull up the vinyl and it's underlayment prior to installing the hardi. Tile industry guidelines specifically exclude all 1/4" underlayments (plywood, luan, masonite, particleboard) from being in the subfloor sandwich. It's not uncommon for the screws to go through the backer, into the underlayment and when the screw hits the subfloor, prior to starting into the subfloor, the partial turn of the screw causes the 1/4" ply to ride up the screw creating a void between the underlayment and the subfloor. Once the screw head hits the hardi, it stops and the underlayment never gets joined back to the subfloor. Now you have tile supported by a non structural cement board panel and 1/4" plywood. Not good.

Pry up the underlayment panel with the vinyl still attatched. Where you se the vinyl stretch as you pry is the location of underlayemnt panel seams. Slice through with a utility knife at those locations and you can carry out the old vinyl and underlayment in 4x8 sheets. It actually goes pretty quick.