I hear your concerns about the Vollmer back injury, although there are a couple of things that tend to make me feel a bit more positive about it:
1. Belichick kept him on the active roster, even though Vollmer only started 5 games all season. That's basically 25% of the season, with a huge gap in between that could have been used for another roster signing.
That tells me that either Belichick had inside knowledge that the back injury should have not had quite as long of a healing phase, or that he values him so incredibly much that he was willing to wait anyway. OR maybe a combination of both?
2. With almost 6 full months to any significant contact in TC, Vollmer may have had a lingering issue that simply needed some time to heal. With 1/2 of a full year, he should be getting the proper treatment and fulfilling the healing process.
3. This is even greater speculation, but I would think that if it were 'that' bad, Vollmer would have had some type of surgery on it. There's the fact that as of right now, it's not bad enough to warrant surgery, and even if it is, the surgery would likely alleviate much of his discomfort.
All of this is just personal opinion of course, but those are 3 things that come to mind when thinking of the Vollmer back issue. I personally don't think it will be as much of a problem as some do. Even if it is, he should still be way more effective than he was last season. There are players who play with nagging injuries all of the time and are still somewhat successful; especially the big uglies.