I don't want to get too far into this - IMO there's also a big difference between doing something systematically and regularly and in the heat of the moment too.
A lot of it has to do with what does a person objectify? There are a lot of animals most of us consider objects - tools with a purpose to somehow help us or enhance our lives. The chicken we buy at KFC or the hamburger we get at Burger King.
I used to rodeo and train horses for a living and to me those animals were objects. You weren't deliberately cruel to them but at the same time they were objects with a purpose - if you didn't feel that way then the odd calf that hit a rope wrong and broke its neck in practice would drive you out of the sport. And if you're racing horses the chances of an animal breaking a leg is much greater if he's running with everything he has on the track instead of with his head down in a pasture. Plus it would be pretty tough to sell a horse if you let yourself get emotionally attached to it. Interestingly, now that I'm not in the business and just have horses to trail ride my emotional attachment has increased a bunch - don't have to worry about selling them.
Dogs (and cats for a lot of people) are different for many of us - they're more a member of our family so when things are done to them we're more offended.
So while I don't know exactly what goes on in Vick's head, the mere fact that he fought dogs doesn't automatically qualify him as 100% reprehensible. I have a harder time getting past drowning or hanging a dog rather than just shooting it in the head since that is unnecessarily cruel and indicates deriving pleasure from something else's pain. But the simple fact that he viewed dogs as property - something to be owned and used purely for his benefit - doesn't in and of itself make him sub-Human.
Of course dog fighting is illegal and we're a nation of laws but whether he feels himself above the law or not is another issue.