The way i see this-which, mind you, is from a relatively casual player and is rudimentary compared to some other perspectives- is that Vengeance is only better than Rage where you need that little bit of extra damage that Rage doesn't get in a burst. Problem is, the sustained dps of vengeance is obviously slower. So, you have to factor in the idea that in a longer fight, will vengeance's extra damage pay off? Or would it still be faster to go Rage and just do a part of the initial burst sequence twice? In a "longer fight," one might get two force screams, etc. that could be used to get that extra "umph" to finish them off after the burst.
So, i guess what i'm saying is:
Vengeance is better than Rage if one enters a fight in which the length is longer than Rage's initial burst and follow-up, but not so long as to where the Juggernaut's mildly more powerful abilities are coming off the CD.
Not factoring in the idea that Vengeance is much easier to heal through, being drawn out with its weak dots...it does have the enhanced utility, but you can grab the better points there in a Rage build anyway, if you felt like it.
Now even after saying all of that, i'll be the first to say I like Vengeance more than Rage for everything, as it is more fun and challenging, and just "feels" better to me. But in order to be better than Rage, you need to have a very particular situation where they aren't healing, and taking enough damage to survive one burst but not a burst and a half.
If that makes any sense whatsoever...That was a lot longer than intended >.<