Desean Jackson had 3 concussions last year and the year before had a few. Thats not on accident.
Troy aikman had to quit football due to having them.
Some guys are just more susceptible to them, and I do believe the harder you are hit can increased the chance of a concussion. That could be a semantic type argument though, the way you are hit can contribute to it being harder.
I also believe that being small and being hit by big guys can increase the chance. Eagles are loaded with those guys. Or when you run all the time as a small qb, the chances of being hurt go up...whether concussion or other injures.
I also thought that hit on Vick was weak. He's soft compared to other NFL qb's. JMO..