Since taking up tanking (I know the picture is dps gear... shaddup), I've become obsessed with threat. Thus... I love Vigilance.
Using it properly is another matter. So here's some short thoughts based on experiences so far.
Vigilance does three things, and to me, it's a required tanking tool:
1. It reduces the target's damage by 3%. May not seem like much, but it helps out on those AoE meelee guys like DKs and Pallys.
2. It transfers 10% of the person's threat to you. Again with the DKs. They AoE to the point where you can't even see anything. Well, having 10% of that, plus Deep Wounds, Thunderclap, Damage Shield, and Shockwave all working at the same time is gonna generate bunches of threat on multiple mobs at once. Win.
3. Refreshes the cooldown of Taunt everytime your friend is hit. So, even if they are pulling aggro off you, you can pull the mob right back. Over and over and over... until they get the picture.
Vigilance does not go on your healer. It might go on your healer, but it's unlikely the best place... though it is the most tempting place. Most of the time, at least in Wrath 5-mans, the healer isn't going to out threat a Warrior tank. The threat they generate isn't enough to transfer well. And they rarely get hit.
Vigilance works best when it's on a party member who tends to get hit a lot and tends to generate piles of threat. So your first choice is gonna be your top melee DPSer. The benefits include getting a nice chunk of their threat, while increasing that fine line between good dps and dead dps by 10%, lowering thier damage, AND refreshing Taunt regularly so that if the healer DOES pull aggro, you aren't left hanging with a cooldown (macro Intervene>Taunt>Shield Slam>Devestate).
If you're like me, you PuG a lot (at least in partial PuGs, anyhow). So when you have macho-boy thinking he's all bad cause he can pull aggro off the tank, slap this on him. It might hurt is ego a bit, but life's not perfect. It's not an ideal solution, but as a quick fix it works. (Sneak it in during a fight. Most of the time they'll never notice.)
As a second choice, I'd pick a Lock or a Hunter with high dps. I haven't tried it, but if it'll work, sticking it on a pet or minion that keeps pulling aggro would be great, too. (Not sure if this works, but will try to confirm.) With thier big dps potential though, I go for the DK, Pally, or Hunter first.
So get your protective eye on someone. It's a great ability, easy to take when you spec, and it can make your life as a punching bag MUCH easier!