Vinnie knows as much about running that offense as we do.
Assuming Vinny is the best choice for holder (and I'm not assuming that, but let's say he is for the sake of your point)... does BB, living in fear of a worst case scenario of Brady going down and Cassell is unable to immediately come into the game, decide to get by with one fewer roster spot?
Or, knowing that the chances of Brady going out are low, and that Vinny could get by at QB for a quarter or two until the 4th quarter when Cassell comes in, does he post Cassell as the 3rd QB, give Vinny the opportunity to hold, and retain his roster exemption for one of his QBs?
I wouldn't spend too much time worrying about whether either Cassell and Vinnie are anywhere near Brady's level of knowing the offense. If Brady goes down in a game, its going to be short passes and a heavilly run oriented offense that takes very few risks.
We're probably screwed if Brady goes down anyway - why not take the best holder available for the games when we have him, as well as a roster exemption (if indeed Vinny is the best guy for the job)?
That's the bottom line - BB will use the best holder he has available. It's too important a job.
He won't let a worst case scenario decision about whether Cassell or Vinny should be the emergency QB dictate his decision on something that is sure to affect many important point scoring plays in every game