In all likelihood, it won't make much difference what they do with Vince. He isn't likely to become a great reliever, but he isn't going to suddenly become a good starter either.
But we already know what he is as a starter. A 1 WAR starter is one win better than a replacement player. Maybe whoever takes his spot in the rotation will be a replacement level pitcher, and we'll lose that win, but maybe a Moore or a Howard could be better. There just isn't much to lose by giving up Vince's spot in the rotation. And if he can become a good reliever, well we know we need help in the pen. The best outcome might 4 WAR between the last starter and Vince in the pen. Not likely, but more probable than Vince in the rotation and someone else n the pen.
We all know that the only way the Phillies can contend is if a lot of things go right. Giving themselves a couple more chances at some things going right, like one of the starters they signed doing well and Vince finding himself as a reliever. is a better strategy than making the team look more like last year's team.