I've moved my toilet directly over the 3" main drain (not a vent though, vent will be 1&1/2" out of the side of the 3" tee) during a remodel of the bathroom. Can I connect the closet flange directly to the 3" drain w/o a bend? Thanks in advance

I'm rebuilding two bathrooms starting from the bare studs and subfloor. I will be installing tile flooring and tile walls in both. Occasionally I run across a recommendation to use a vapor barrier behind the cbu (1/2" hardibacker on the walls) and in between my subfloor(2X6 tongue & groove decking) and underlayment (3/8" plywood) "to prevent moisture from getting behind/under the cbu hardibacker and causing damage." This doesn't make sense to me because... if water gets past the tile, then past the cbu hardibacker, it's gonna go right through any vapor barrier because of the hundreds of screw holes in it! So what's the real purpose of the vapor barrier in this case? Are we talking condensation on the back side of the hardibacker that may form? Or maybe actual "vapor/steam" from a hot shower permeating through the walls and floors? Any ideas?