Eh, I don't know.
Plummer isn't even 220, and he's taken quite a few hits over his career. Arizona didn't exactly have the best of offensive lines. Only in two of his seasons in Arizona did he not have as many, or more, sacks than he has had in two combined seasons in Denver.
In addition to that, most of the 230 QBs around the league are 6-4 or taller. Vick's about 4 inches shorter than the guys that size. 4 inches can mean an easy 10 pounds itself I'd believe. IMO, if he stays in a 215 area he'll be just fine.
Besides, while his straight ahead speed probably wouldn't see much effect, you'd have to wonder about his lateral quickness. Would he be able to dodge the same hits he has if he had another 15 pounds? It's debatable he may take more hits with added weight, thus negating the bonus it may bring him.