Have an 18 x 24 carport that I want to raise the level to make an art studio for my wife. The previous owner did not raise it much higher then ground level. Plus I don't think he put a vapor barrier under the slab because of the moisture on humid days that wick to the top of the slab. The rear of the carport has an enclosed tool room of about 8 x 18 which is raised and won't need additional cement topping. I probably will use a 2 x 4 to frame around perimeter. Can I put a vapor barrier on the old slab? Or should I scar the old surface and apply a bonding agent so the new slab will not shift? I don't believe I can do both??

Before you do anything you need to confirm is the moisture was wicking or condensation. While the concrete is dry duct tape a clewar plastic sheet or bag over a 1' area. Wait 24 hr, if there is moisture inside th bag then you do have a problem. You can pour on top of the old slab but you can't add a vapor barrier

If you have adequate footings, you can hammer drill 3" deep holes for #5 rebar(5/8") 6" long and set them 3" above the existing floor and put them 3-4 ft apart. Put down vinyl sheeting, 6 mil or more. Pour the new floor, the vinyl stops the water intrusion and the rebar pegs keep the floor from shifting.