Remember, as well, that Yoda knows that Vader is Luke's father and Luke doesn't and he is, perhaps, concerned that Vader will tell Luke and Luke won't be able to handle it.

In ROTJ he expands:

Yoda: "Your father he is. Told you, did he?"

Luke: "Yes."

Yoda: "Unexpected this is. And unfortunate."

Luke: "Unfortunate that I know the truth?!"

Yoda: "No! Unfortunate that you rushed to face him! That incomplete was your training! That not ready for the burden were you!"

Luke: "I'm sorry."

I reckon Yoda didn't feel that Luke was ready to withstand the physical and psychological onslaught that he knew Vader would subject him to. Add to this that, in Yoda's mind, the return of the Jedi is the only hope for the galaxy and you have a very good reason for Yoda to be against Luke going to help his friends.

Anyone agree with that assessment?