vanish aura is applied instantly, but stealth aura is given after 3 seconds, BUT, vanish aura = stealth as well
so to answer your question: they should see you for 0.2-0.3 seconds with vanish aura as you see in in my other comment (
)
Pay attention to his focus target (the mage). he vanish, but mage still sees him for like 0.2 - 0.3 sec, even though he is in vanish.
if you're confused why his action bars does not change, it's because he is using bartender.
NOTE THIS:
This 0.2-0.3 sec stealth detection is ALSO applied when you see random people in stealth. this is why it's very hard to find stealth players on pwow