#15 - July 21, 2009, 3:13 p.m.
Vanish not working on city guards is intended, because we do not feel that it is a good thing to provide an ability to grief in cities and be able to escape with little to no retaliation.
You are of course free to infiltrate enemy cities as much as you like, but be prepared to defend yourself (or flee in the old fashioned way) if you are discovered.