I think it depends on where he is falling. If he gets past Arizona (Which he won't, but for the sake of argument, let's pretend), he may well fall into the Cowboys' lap. That's tremendous value at the 18 slot. They could draft him and groom him for a couple years, for when after BP leaves (Think Pennington in NY) Likewise, what are trade-down possibilities? Whether you are interested in VY or not, that's a great position to be in.
Now, would you trade into the top 10 to get him? I don't think so. That's giving up a lot of draft picks for a player with some legitimite question marks. VY has potential, but potential and a dollar will get you a cup of coffee.
If he falls into that ~15 range, do you move up a few slots? This is where you get into that gray area, imo. What are the teams current needs? Does the 18 slot need to yield an immediate starter? Do you have faith in Henson (I don't)?