A vapor barrier does not shut down any perspiration. It stops insensible perspiration (involuntary water diffusion through skin) from evaporating. If used in warmer weather it could cause more sweating. Without a wicking layer, or even with one, your skin may feel clammy from the moisture. The main purpose of a vapor barrier is to stop moisture from freezing inside sleeping bag insulation in winter. Expeditions of the past have failed due to this.