moisture barrier

Hi - hoping someone here can help since I've received 5 different opinions from 5 different reps at various wholesale flooring sites. We're looking to install 5mm luxury vinyl plank flooring in our basement, which has a concrete subfloor. This is the floor in case it helps...

I stupidly framed out my basement and put up drywall, which looks great. I then realized that I should level the floor - it drops 3-4" over 12' and it isn't flat, either. I want to use concrete to level the floor so that I have more flooring options. My drywall ends ~1" off of the floor so that...