Originally posted by MythLord
He didn't. The entire thing was all Palpatine's plan. Both the comic and the novel note this.
Then why was Vader unaware of Galen's presence on Kashyyyk until after the lightsaber had been TK'ed out of his hand? The only logical reasoning Vader could have had for taking Galen on was to either use him as a servant to carry out missions for him or to overthrow Palpatine.
If it was the former I don't see why Palpatine would have addressed Vader to warn him against betraying him again.