The real problem with vanish is that the majority assassins seem to abuse strafe as soon as they pop it. You'll occasionally see that some even start strafing before it fires. If a GM is able to see stealthed players I can guarantee simply watching stealthers on the main bridges every evening would result in a ton of warnings/bans for strafe abuse.
Assassins aren't supposed to be found easily after they vanish regardless of whether or not they are "cheating" with strafe. You seem to be implying that you think they should be. There are plenty of problems with the archetype on Phoenix, and vanish is super frustrating, but I don't think completely nullifying their only "defensive" RA with a potion is a good idea.