A vapour control layer is a thin sheet installed on the warm side of insulation. Its job is to slow down the movement of water vapour through the wall. The mechanism is simple: a continuous plane stops vapour from moving through it. But "continuous" is the keyword. A vapour barrier with one hole is not a barrier anymore. Ten holes, or a hundred, and the performance fails. A bathroom is the room that most wants to put holes through a vapour barrier, which is why this detail gets more attention than most people think.
Every electrical box, every plumbing penetration, every recessed light or recessed storage niche is a hole through the vapour control layer. Each hole has to be sealed, taped or caulked to restore continuity. If the holes are not sealed, the layer is not functioning. Moisture vapour moves through the unsealed penetrations as easily as it moves through no layer at all.
A recessed light fitting in a bathroom asks for a big hole in the vapour barrier — the fitting has to be set back into the wall cavity, the hole around it has to be large enough for the fixture, and that hole is now a direct passage for moisture. A recessed niche for storage is even worse — the entire recessed area is open to the inside of the wall. Each of these details is a decision made during framing that affects how the vapour barrier works.
The vapour control layer is different from waterproofing. Waterproofing is a layer in the wet zone — behind the shower, tiling the floor — that sheds water outright. It is thick, visible, and its job is to stop liquid water. Vapour control is thin, usually invisible once the wall is closed, and its job is to slow water vapour. They are two different layers doing two different jobs, and they are routinely confused. A bathroom has both — waterproofing in the wet zone, vapour control in the walls.
The material is usually polyethylene film, sometimes poly-coated kraft paper, sometimes a variable perm membrane that changes its properties based on humidity. Each material has different permeability and different installation requirements. The seams have to be lapped and sealed. The penetrations have to be sealed. The detail at the junction with other components has to be thought through.
The layer is installed on the warm side of the insulation — facing the bathroom, after the insulation is in place. This is a framing-stage detail, done while the wall is open and before the interior is closed. Once drywall is up, adding or fixing the vapour control layer is nearly impossible. The sealing of penetrations and the continuity of the plane are details that have to be right the first time.