If your installing a shower should you put a vapor barrier behind the cement board or seal the cement board with redgard? I've heard both is bad, so one or the other. One wall is on an exterior wall and has a vapor barrier already.

Your insulation vapor barrier should face the warm side of the wall already. Never make a double vapor barrier in any walls. Butter the seams on Wonderboard or Hardiebacker with the same modified thinset you use the set the tiles.

OK, thanks. I didn't think sandwiching the cement board would be a good idea but my tile rep said to do it.

Tile rep are probably great at helping to find the best tile but not always so great with installation. Advice. You should embed some fiberglass drywall tape in the thinset when you butter the seams.