Not sure if anybody has posted about this or if this is a known bug, but this is how it goes:
1. Attack a player or guard of a major city (like ironforge), then run away and try to vanish.
2. You will notice that the guards do not stop pursuing you and they will bring you back out of stealth.
3. Occasionally you wont even go into stealth if they are too close.

So, in conclusion, there is an obvious issue with vanish regarding the resetting of these NPCs. Whether or not this is typical of all PTRs or just an isolated incident, I don't know. But, these are the facts, take 'em or leave 'em.