It is more about engaging in an illegal gambling activity than killing animals.
The Feds and the NFL can target gambling with the greatest ease. Less so with the dogfighting, because its lack of prevelance with either.
Public opinion will be a difficult hurdle, but so will the NFL Commissoner. When you have someone like Vick being a prime mover/funder of an illegal gambling activity, it brings into question how much the integrity of the NFL may be compromised. Vick's association with other gamblers and his access to inside information on the NFL, does raise questions as to whether more than just dogs were involved. It also made Vick subject to blackmail. Even the appearance of an impropriety is enough to get him banned for life. I hope he is.