Well, vampire venom is presumably responsible for the regenerative healing. If foreign venom performs the usual regeneration, but does it incorrectly, then the original regenerative mechanism might not recognize that there was still damage to repair.

This suggests that a venom scar might be repaired by inflicting ordinary damage on the same spot. But there would likely be a time limit for this procedure, since the original regeneration system would update to the new shape. (If it did work this way, the time limit would almost certainly be equal to the duration of turning.)