You are 100% correct! Essentially, what you have to do is vanish at the exact second your opponent presses blind. I think doing it just by reaction time in itself is impossible, the spell has no travelling time, there is no projectile coming towards your or anything.. It's just instant. So you somehow have to instinctively know the exact moment when that spell is used and just klick it out of a gut feeling. Good indicators are character models turning and opponents walking towards you (if you know the minimum distance of the spell) and so on. If it doesn't work out you gain literally nothing, you wasted your most important defensive and offensive cooldown and you look like a completely tool. This mechanic wasn't even intended by the game creators by the way. Vanish gives you a .2 second immunity to spells apparently, some people noticed that and tried to abuse it. I think the best thing about it is that if you vanish a split second too early then you actually get vanished out of the blind, which isn't supposed to work due to mechanics, another malfunction.
Sounds complicated maybe, after some searching I finally found a good video where it's shown done to perfection at point blank range. Luckily you can pull this up to HD to see very clearly what's going on. If you pause at about 7 seconds, where it happens, you can see the hands of the other character "glowing", that is the cast animation. So the instant the other character model started turning around, this YouTuber clicked vanish in anticipation of blind, it worked out. You can see him entering vanish when it says "Exit Combat" on the screen. It's of course most fun if done in a duel, because it's the best way of establishing: "I know exactly what you're thinking at any second".
This second video shows it even better I think. Clearly that guy is a million times better than me and has god-like reflexes, but even he needs a clue to vanish the blind. Not for the second operation though.
It's one of those ultimate "style on you" scenarios where if you do it wrong (99% of time and players) you look like a complete fool and it'll also make you lose, but when it does work out you'll turn heads for sure.
As you can probably tell I wasted my youth on this piece of junk, but I can't stay I don't still love it. Never felt like it was in vain, this was probably the most fun I've had with videogames in my life and I'll likely never replicate it. From what I can tell WoW is hot garbage right now, but I won't touch MMORPGs anymore anyway
