Yeah tank vanguards are effective in lowbie PvP but at the 50 lvl vanguard tanks fall behind the tanks classes.
The Vanguard tank is build around passive mitigation, hardley any cooldowns. Unfortunatly the majority of this mitigation shows up as shield mitigation, and armor, which considering the amount of tech/force attacks and internal/elemental damage that pretty much all classes (exception is sniper/slingers which have mostly ranged attacks) have in abundance. Jugg and sin tanks have much better cooldowns that fare far better agaisnt such attacks assuming the player is competent.
Vanguard tanks have decently higher dps than juggernuats but our utility is far less than theirs and about the same as Sin tanks. But considering how many crowdcontrol options available to Sins and a lesser extent juggs the vanguard tank again falls behind. For such reasons a lot of ranked teams wont even take vanguareds tanks opting for the other two classes. I have been turned away by ranked groups multiple times solely becuase I'm a vangard and theyd rather have a gaurdian or shadow even if they are lesser geared than me.
Vanguard tanks can compete competitively endgame lvl but we have to have much better gear and skill to bring ourselves up to par with other tanks who are less skill and gear intensive. Not to mention how aweful a lot of the trooper tank pvp gear is.
I somewhat disagree with this, but not having a level 50 vanguard at the moment (last one went poof), I'm not in a good position to argue the point.
What I can say is that you have to change your fundamentals for gearing as a vanguard tank in PvP. This extends to the skill tree as well. Shielding attacks is next to worthless as you can't shield if they score a crit and shields don't work on elemental and force attacks. Since crit ratings are high and elemental/force attacks are the main source of damage in PvP, you need to start focusing on defense, offense and overall utility to excel.
It use to be that a good PvP spec was a tanking hybrid that utilized much of the tanking tree, and went up the assault tree for the Gut ability. Many of the vanguards attacks are elemental in nature or bypass armor, so your in a good spot from a potential dps standpoint. Gut simply gives you yet another on demand internal damage source that you can use to leverage high impact bolt. Keep in mind that, while you can spam the Gut ability, it's damage isn't all that impressive and it's only really there to provide HIB support. Your focus should be on mostly offensive stats even thought you're a tanking role, with crit and surge being of paramount importance. Worry about defense rating and power after that. Shield and absorb rating is relatively worthless as these only protect against about a third of the incoming damage you're going to take. Not wing there, or interrupting an opponent are more important tactics. Use storm and harpoon to your advantage in this regard. There are plenty of ways to peel opponents off of a larger groupso your team can beat them down. Remember, warzones aren't about the 1v1. Use guard on a healer and protect them. If they're any good at all, they'll look out for the guy that's taking 30% of the damage for them.
Al that said, I'm not sure where that spec stands these days. Since starting over on a new server (and switching factions) my powertech (Empire faction vanguard equivalent) has since been deleted. I'm currently sitting at just shy of level 40 and have yet to really set food inside of a warzone. I'm just working on getting some 50s up and running right now so I can start farming credits and unlock the cool toys. But I don't think vanguards are in nearly as bad a shape from a PvP standpoint as dps mercs/commandos are, and are just about as viable as any other tank if geared and spec'd properly for PvP.