How to pour concrete over existing slab?
To address the vapor barrier aspect:

With some materials you have to be careful about sandwiching them between two vapor barriers which can trap moisture - but concrete is not one of them. Adding a vapor barrier before new concrete is a good precaution if you're unsure if one exists already.

Closed-cell foam insulation can itself act as a vapor barrier (I recommend taping the seams and / or using spray foam to seal it all up).

So whether you use plastic sheeting (polyethylene I believe) or insulation, you can lay it on the existing concrete and then pour over that.