It seems that very normal, healthy people can be trained to "turn off" the empathy switch. We know this from historical example, and there are of course several social experiments conducted in the 60's that purport to show this (e.g. Zimbardo's Prison Study and Milgram's Shock Experiment).
I have no idea what Vick's standard might be, or whether his lack of empathy for fighting dogs was natural or learned, but I do think it's possible for him to have a dog as a pet and treat it well - so long as he views it as a member of the family, so to speak - despite having such a deplorable past record. But I'm certainly open to being convinced otherwise.