If Vick plays this year (which is in doubt), I doubt he will be in football shape. It may take him an entire season to get that way and his top speed may be gone forever. I would say it would take him a while to get used to reading defenses again, but when was he ever able to read defenses in the first place.
Personally, I am not worried about him going to the Fins. If he becomes their primary QB, he will not be all that effective and hinder the progress of their young receivers (see Roddy White and how he has transitioned from a first round bust to one of the league's better receivers the last two years after Vick left). I was never a fan of Vick because he was a lot of excitement, but he was a limited QB whose production didn't really match his excitement or hype.
I remember when the Pats played against him several years back and I think Vrabel saying that when they studied film on him that they laughed because he ran the same 4 plays over and over again. My guess is because he couldn't handle a full playbook.