The build-up of a warm flat roof starts with a good vapour barrier followed by insulation then more decking or just the roofing materials depending on if you want the roof soft or solid.
A vapour control layer helps you shield your building from the consequences of condensation. Condensation is generated when warm moist air rises and condenses into a liquid on contact with the cooler areas above the insulation. The notion behind a vapour control layer is to install it on the room side of the insulation so it stops the passage of warm moist air entering the structure.
This video shows me testing the vapour barrier when we did a job late last year. A primer for the vapour barrier is recommended if you are going to glue down over the top of a surface that not sound and should also be used if its cold. It sticks very well sometimes too well if you let it touch itself you have to throw it away as you just won’t get it apart.