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Alchemists can make Potions of Invisibility. However, all players of any class can stealth. Crouch [default = Ctrl] and sneak around. For all intents and purposes you are stealthed. In PVP you will be invisible to other players and will only begin to fade in if they get close to you. When this happens, you'll see the iris begin to open slightly, meaning you're being detected.
I'm guessing you're referring to Clouding Swarm, which is a morph for the vamp ultimate. I haven't really played enough vamp to try out that ability myself though.