There's no possibility of any phthalate synthesizing itself as part of the vinyl chloride polymerization process. Can't happen.
Certainly PVC resin could be contaminated during compounding or subsequent compound processing (extrusion, etc.) if run on machines that were not cleaned from the previous batch, which was phthalate plasticized. My understanding is that the industry norm, except for specialty makers, is not to clean such machinery between batches.
I'm told that for compounded, shape-processed flexible PVC to be phthalate free, it has to be run on lines that only process phthalate free materials.