Blizzard has said on numerous occasions that Vanish has never truly worked as intended. And you hit the nail on the head, it's a poor game mechanic.

As to whether or not they are really fixing it in Cataclysm ... I'll believe it when I see it.

What vanish is supposed to do, mechanics-wise, is put the rogue into stealth, take the rogue out of combat, and cause anyone targeting the rogue to drop target.

Further mechanics do not allow players to cast certain abilities when they do not have a target, but because vanish runs into latency issues, it allows those spell to continue casting (or melee swings to continue) after the rogue has vanished and should no longer be targeted.

Other spells, like fade may have similiar issues where timing is very very important. I think the difference is that vanish is a game breaker; if a vanish fails, a larger portion of the rogues toolbox is now out of reach. that being the ability to open again with the attacks only usable when in stealth. Fade is used to midigate damage primarily, which is why mr. Christian argues that it is only fair vanish does not allow rogues to become immune to damage, because that's not the purpose of vanish.

Vanish is meant to put you back into stealth.
It increases stealth to such a level for a few seconds you will not be seen.
However it is broken so:
- Attacks in the air still hit and break it
- Attacks made during the time span of clicking vanish and the effect occuring still hit, breaking it
- Targeting is not dropped all the time though the rogue has a stealth level much greater than any detectable level and may be hit by an effect used after they vanished including standard melee attacks.
- Pets of all kinds do not drop target and will give chase, hit, and remove stealth

Remember vanish is a hinge ability.
It's an advanced form of feign death.
It used to have a reagent cost on top of the cool down, now dropped to 3 minutes, because it was intended to be a strong ability.

In the case of a resnare, note that vanish can break while the animation is running thus giving none of its benefits but burning a 3 minute timer.
In comparison to a heal, well if you cast it and lost the health due to a hit, that's normal mechanics. You gained the benefit of your ability. Vanish breaking is often complete happen stance and does not give you the benefit of the ability.

As to how it's known to be broken is that Blizzard has admitted that it is broken and has been for a very long time. They have been trying to fix it but have been wary of how they do so or it will become over powered.

Vanish is just broken.

Example.

Rogue vs. Hunter.

Fight starts. Rogue wants to vanish and get away. He uses Cloak of Shadows(removes any DoTs and Hunter's Mark) and sprints away. Uses Vanish. He is now supposed to be safely out of sight. (unless a Shot is fired knocking him out of stealth).

However, The Hunter's Pet magically still can see the rogue even in stealth. The pet follows, attempting to attack.

The rogue must run away until the pet stops following to avoid an auto attack, effectively breaking stealth.

I did some research and this was supposedly addressed in the bugs section of the 2.2 Patch notes. Not sure If it that fixed it or not however.

Not to mention Vanish has a 3 minute cd (2 when talented). Fade has 30 second cd (15 sec glyphed and talented) and mentions nothing about being immune to snares for any sort of duration. The point of being in stealth is to not be targeted and therefor not being able to be hit by anything.

If Blizz wanted to lower the cooldown to say 30 sec(?) then I'm perfectly ok with getting knocked out of stealth by a Hunter's arrow a full 2 seconds after I pop Vanish. But to use my main "Get Me Out Of Trouble" button and have it fail a lot just seems unfair to me.

Chase made a great point - What's the point of using Vanish if I'm just going to be unstealthed by something that's not player controlled? (You fail if you try to Vanish with a DoT on you) Then Vanish becomes as useful as the Sentry Totem.

Thanks for the great answers.

I think it's odd you guys call it a bug - seems to me it's more a case of a poor/unbalanced mechanic - but glad I understand it more clearly.

Rogues have always been a very "technical" class in PvP compared to others - you play a lot more on the fringes of of the code - Vanishing Death coils, using the ability to get out of combat to allow restealth, use sap etc.
As a result I suppose it makes sense that something like this would be seen as a "bug" rather than just a balance issue. I think most other classes would not call it a "bug", but I think that's a matter of class culture - I don't mean this in any kind of derogatory way.

No you're totally right. People who don't play a Rogue don't understand how frustrating it is when Vanish doesn't actually make you vanish. They just /laugh.

Blizzard calls it a bug. It's a bug.

As a Warlock I'm torn. I've spent enough time playing a friend's rogue to agree vanish is broken and needs fixing but I think this may be too much. 3 second immunity means no chance to aoe and bring you back out, as a lock if a rogue is able to open on you twice the fight is lost (assuming they are good). I also see using dots to force a cloak if they want to vanish as a viable strategy. Rogues already have a lot of tools against squishy casters, this tips the balance too far.