i have a enclosed back porch with a single door at one end wall, sits on concrete slab when it was enclosed. door was installed with no threshold. can i install the sliding patio door so the doors threshold sits on concrete floor( door is vinyl) or do i have to put wood framing on floor for door to sit on, thanks for any help jim

You can seal the threshold directly to the slab. Use several runs of a good grade of silicon caulk.