I don't think this is as obvious as it seems. Going by a pure numbers game, I think you have to pick 2 of Woodhead, Vereen, and Demps. It wouldn't shock me to see four TEs on the opening day roster next year, and that's not including whoever end up playing FB, whether it be Larsen, Fiammetta (who reportedly needed a year off but intends to attempt to play next season), or someone else. Those roster spots need to come from somewhere, and five RBs seems like a waste when you only use one at a time (and occasionally none) and with Ridley as a pretty obvious lead guy. Most games this year, it's been a rotation of two or three RBs in any given game, not all four. That makes me think that five is overkill.
Now, if Vereen doesn't improve at all over the rest of this season, then the solution is pretty simple. Cut him, sign Woodhead, go with Ridley, Bolden, Woodhead, and Demps. Similar if they decide Demps can't play, but that's probably unlikely considering they'd have to resign Woodhead before they'll see Demps on the practice field.
But cutting Vereen in favor of Woodhead isn't as obvious as it might seem to come. Against the Jets, it was just about an even split of snaps between Vereen and Woodhead until Woodhead played and Vereen didn't on the last two drives. In the Rams game, the snap counts were basically equal. I don't know about this game, but I think the snaps favored Woodhead quite a bit. Either way, it's clear that Vereen has at least earned some of the snaps that would have been Woodhead's a year ago. We just don't know if part time third down back is Vereen's ceiling, or if he can continue to grow as a player. He's definitely got the body to be more of a well-rounded player than Woodhead, even if he hasn't shown that yet.
Ideally, they'd be able to sign Woodhead for a deal with not much guaranteed and then let everybody fight it out. Make Vereen show he can step up into the full time third down back role and make Demps show he's an upgrade at kick returner, and have Woodhead competing as well. But I don't know if there will be a market for Woodhead to sign somewhere where he won't have to compete for a job, which would make a lot more sense for him financially. The Pats are probably going to have to commit to either Woodhead or Vereen during the free agency period, meaning what we see from Vereen the rest of the season is very important.