I've liked this kind of wound/vitality system (with your Con subbing in for a separate pool of "would points") aesthetically. The side effect of this is that you end up with two tracks of healing magic: proper healing magic that heals Con and light "hp-only" magic that improves stamina.
But maybe this is a good thing? Cure Light Wound might heal 1d8+x hit points and a single point of Con damage. That would actually be a "light" wound!
All of a sudden you have my attention. I like this, quite a bit.
And for all of you worried about the cascading math of Con modifier throughout your sheet, I gather it's a lot less extreme in Next. You've just got your Con - that's it. What used to be a Fort save? Con score. And so on.