*shrug*
If I were using the GM hat I would say that you could manage it if you found someone with a +4 Con belt and would be willing to train you, use retraining rules and throw in a little extra for use of the belt during that time.
Alternatively Cord of Stubborn Resolve with a stat increase from when you level.
There's an even more complicated possibility, if something along the lines of this sequence were to happen:
1. Character with a Constitution score of 12 gets a Belt of Mighty Constitution +4 and wears it for several days.
2. Character levels up and takes the Raging Vitality feat, then takes off the Belt.
3. Character begins raging, and their Constitution is now 18.
4. Character takes 2 points of Constitution damage from, say, a poison. Their Constitution is now 16.
The character still qualifies for the feat, but if they didn't have it, they wouldn't qualify. It's clear that they would lose its benefits if they stopped raging and started again, but would they lose them immediately?
Moral of the story: Don't make feats that give numeric bonuses to a numeric score that they have as a prerequisite.
Actually, there is no problem here, since damage to an ability score explicitly does not actually reduce that ability score.
Ability DRAIN might cause this problem, though.