also when luke seen his face in vaders mask when he went into the woods was yoda tryin to show luke that vader was his father and luke didnt understand it then.I always wondered it and yoda also said "You only take with you what you need" and said "NO GUNS you wont need them".I have always wondered this
Ok: a couple things.
1) Yoda didn't conjure up the Vader specter. That was a manifestation of the Dark Side power of the cave.
2) Luke saw his face in Vader's mask because it was foreshadowing what he would become if he followed the dark path. Remember, Yoda and Ben were going to tell Luke the truth only
after his training was complete. Why, then, would they risk jeopardizing everything by trying to slip him the info sooner?