Vanish will never work flawlessly all of the time, there will be times where an enemy attack will register before your vanish on the server, but not on your client.
Unfortunately we can't trust the client 100% of the time, but we try as much as possible to offer the smoothest and most responsive game (from the player's perspective) as possible.
There are going to be times where actions will appear incorrectly ordered in the combat log.
There will also be times where a vanish will break immediately.
The biggest causes are going to be improper use (it isn't a fail safe get of out harm free ability), or the server isn't processing the actions in the same order you see them due to the tubes of the internet, and the latency they provide.

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