Y'know it's obviously not ideal to throw 3/4 or sidearm, but sometimes unorthodox deliveries can help, especially if they are quick or are tough for defenses to anticipate/read. The thought is that he'll get some balls batted down and he'll certainly get more batted down than if he threw over the top, but he's 6'5" and does a lot of throwing on the move, so I don't see it being a huge problem. We saw how Cutler got 2 in a row batted down with his "perfect delivery" in the Senior Bowl (BTW, how can a guy be that cocky after a 6-19 performance).
It might not be much of a problem at all, Jerry Gray said in a McClain article that to stop Vince he would probably have his DE's take wider angles to the QB so as not to get Frostee Ruckerized. That means passing lanes are gonna be a little wider than on average. I'm not saying everybody is gonna do this, but it gives you an idea how his running ability makes throwing the ball a little easier for him.