Originally Posted by
Verain
On live, vanish will often -combat instantly. But there are times when it does not. As an example, assume you are in a Blizzard. You vanish, and take periodic damage over the next two seconds as you leave. The three seconds (glyphable to five) are up, and your vanish fades and your stealth activates. After a bit more time, combat drops.
With the glyph, you would have popped visible whilst still in combat.
There's plenty of reasons to take this glyph, but there's also reasons not to.