Insulation, Radiant and Vapor Barriers - Vapor Barriers

I am putting up Hardibacker as my shower surround walls. I have an acryllic, preformed pan. Virginia Residential code doesn't specify that a vapor barrier is required. However, code doesn't always necessarily equate to good practice. I am just planning on putting up a vapor barrier on the two outside walls of the surround. I have the surround down to studs. I am looking at either putting up 6 mil poly or using Red Gard.

First, any recommendations on one vs the other.

Next, if I put up poly, is it installed vertically or horizontally, or does it matter? Second, I've heard/read that after stapling, the seams should be sealed - what is the best way to do this? Seeing as how the screws from the backerboard installation will pierce the poly anyway, do seams need to be sealed? Lastly for the poly, should the it extend over the shower pan flange?

Red Gard - I have heard from some that it is easy to apply, hard to apply from others. How many coats are needed? If Red Gard is applied, do I still seal the seams between walls with caulk?

Thanks for any help you can provide