So, you may be wondering why exactly I'm writing an article on Vanish here in Guardian Spirit. Two reasons, one, it needs to be written since I can't find lore on Vanishblock anywhere, and two, I don't have a 'general purpose' blog.
So, what is Vanishblock? It's a feature of Vanish (and to a lesser extent, Invisibility) that can allow a skilled Rogue (or an insanely skilled Mage) to entirely avoid any one attack for the price of their aggro reset cooldown. The Vanish page on Wowhead makes a reference to the Vanishblock ability in the first comment: "[Vanish] can be used to avoid both physical or magical damage if timed properly." This is, unfortunately, the only reference I could find to Vanishblock anywhere when I went to try and look it up for a Rogue friend; if I hadn't already known more-or-less how to do it, I wouldn't have been able to teach him the secret forbidden lore of the Vanishblock.
We eventually met outside Orgy for some duels to practice with, and it worked exactly as I expected, he could block Pyroblasts, Flamestrikes, Mind Blasts, basically anything he could see coming, either in midair or via my cast bar. When blocked, he would avoid all damage, all associated debuffs, he would prevent me from gaining stacks of Shadow Weaving or healing from Vampiric Embrace, and if the spell had no travel time, it even prevented me from stacking up Eye of the Broodmother. It was as if the spell never happened. What happens outside Orgy stays outside Orgy, I guess. Notably, he could not use it to protect someone near him from an AoE, however if he was the primary target of a spell like Chain Lightning, the entire spell would fail. Targetted healing spells would also fail if blocked, assuming the caster wasn't in the same party or raid.
So, now that I've stated what it can do, I'll post how to gain this amazing ability (and why it's so rediculous). To preform a Vanishblock, simply hit Vanish 0.33 seconds before the ability that you want to block would land. For mages, you want to hit Invisibility 3.33 seconds early (you want to go fully invisible on the same timing as the Rogue's Vanish.)
Now, why is this worth taking the 30 minutes or so to practice with a caster friend? It works on PvE effects. Mark of the Fallen Champion targetting you? Nah, you can just reset your blood meter, thanks. Tear Gas so the raid can pay attention the to RP? Screw that, have a free 20 seconds to stab the old fool, just hit Vanish as the bottle is about to hit the floor. These are the most reliable effects in ICC to block, but I've seen Bone Spike, and Gaseous Bloat blocked too. Since you can't see the target of those in advance, it's a matter of luck, and causing these abilities to fail may wipe your raid.
So spread the word, send your small stabby people here, and let's force yet another Rogue nerf.