...if any? Con 16, current level 3 (haven't picked a feat yet).
I took it at level 5, after a couple of near brushes with death (Con 18).
With Con 16 you get +3 HPs, per level or 9 at level 3.
So that's just a 7 HP buffer you have there, which is cutting it awfully close already.
Waiting for your next feat at level 5, means that at 4th you get a 12 HP boost, which pretty much means you die instantly when you get kicked into the negatives.
So I would say, get it now, don't wait.
When your raging HP Boost > Then your CON score. Usually about level 3:)
That would be the criteria I would use, but not the conclusion I would reach.
His initial CON doesn't affect the number of "Rage HPs", only the CHANGE in Temporary CON from Rage affects the # of Rage HPs.Rage starts out at +4 CON, meaning +2 HP/level. To reach his normal CON score (16) that means 8th level.
(8th character level or HD, regardless of amount of Barbarian levels).
By 8th level, he could also be reasonably expected to have a +2 CON item, especially if doing so synergizes with his class abilities, which it does...
So the 'break point' may even be 9th level.
I would say that it's certainly not 'necessary' to get Raging Vitality exactly when your Rage HP boost equals your CON... after all, many people have played Barbarian characters without this Rage Power at all, and gaining the power immmediately when the Rage HPs = CON score is relevant only when you would otherwise be at 0 hp, something you should avoid normally. So I would say from 8th-10th level is a good range, waiting longer would mean that the Greater Rage kicks in and your Rage HPs shoot up 50% (albeit if you are multiclassing, I would say it's OK to wait a bit longer if Greater Rage hasn't kicked in yet).
There is also the matter of the +1 Rage HP/level (and Fort Save boost, 'higher' death point = CON score, etc) from the higher CON boost that Raging Vitality grants, and that can easily help sway the decision so that you take it earlier (this is the same HP boost that Toughness grants, except it's only temporary in Rage), but that's more of a 'discretionary' decision that I leave apart from when you 'really SHOULD' take Raging Vitality in order to safeguard against the danger of getting knocked unconscious/into negatives and then having your Rage HPs drop out, killing you dead.
If you have Diehard then I would say you have less immediate need to get Raging Vitality, since being in negatives doesn't send you unconscious, albeit Raging Vitality is giving you a higher CON along with higher HP and thus you can withstand 2*(HD+1) more damage.
I took it at 1st level along with power attack.. Getting 6con boost while raging was a no brainer for me.. 3rd level i took iron will..