How to pour concrete over existing concrete slab if existing wood framed wall structure is there? And where to place the new vapor barrier?(I have an existing garage that I would like to convert into an Accessory Dwelling Unit (ADU). The slab is on the ground, so I would like to pour over new concrete to provide the appropriate height difference between ground and top of 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.