I represented a vent free heater manuf and can tell you definitively that no VF Htr can be converted by their listing. As well, given the critical nature of the ODS system, which is the primary safety system for VF, it is not worth trying.
I ran a ventless heater for a couple years in my last garage...never again. Mine was propane, and it DID smell after a while, and the humidity it put into the garage was terrible.