I'd add that this is wrong too. You're probably thinking of Focus guardians and Rage juggernauts (aka. smash bombers/LOL sweepers), which is a DPS spec, not a tanking spec.
In fact, I find that neither guardian or vanguard tanks are highly valued in PvP (although both have DPS specs that are excellent at same). They're actually not bad for objective-based play (carrying the huttball, defending a point), and can help mitigate damage with taunts and guards, but they have trouble actually killing stuff, so people tend to shy away or respec to DPS for PvP.
The exception to the "tanks suck for PvP" rule is the shadow/assassin tank, which is an excellent PvP class. They'll do OK damage and have a big toolkit of abilities that are perfect for PvP: speed boost, pull, push, stealth, self heals, several stuns, guards, taunts, and a strong set of active defensive cooldowns for when you get mobbed.
This post is way off! In competitive pvp tanks are very important for protecting your allies, especially healers. Usually a Guardian/Jugg fills that role, but I've seen Shadows do it all right. True vanguard tanks are exceptionally rare (because everyone is playing assault).
You seem to be thinking of tanking in pvp as a solo job instead of a team support role. In other words, speccing tank so you last longer and don't die as often. That's not the right way to think of it at all.