it's not a 6 game suspension. As long as he behaves himself between now and the start of the regular season, he will be allowed to play week one of the regular season (assuming he signs with someone). He is already cleared to play all pre-season and training camp. the suspension is kind of probationary like "slip up even the slightest bit between now and then and you're suspended til week 6"
But anyway, just because he went to prison does not mean he served his suspension. He served his time and paid his criminal debt to society. Playing in the NFL is someone 99.9% of the country's population probably wish they had the ability to do, it's not a right. He made the league look bad, he made his team, owner, and coaches all look bad. He shamed his family. He killed innocent animals and bold-faced LIED to the commissioner of football's face about it prior to the 07 draft. All of that is essentially irrelevant to his criminal case, but should it be ignored by the league? If the league is setting out to make role models out of it's players and maintain the huge stranglehold it has over the american sports market (due in large part to the rampant corruption of baseball in recent years, mind you), it can't constantly go the way of MLB and just turn the other cheek. Whether Vick is just unfortunate enough to be the one having an example set by, even a 6 game suspension would have been ridiculously light.
He should be counting his lucky stars that he is ever allowed to play again, let alone so soon.