Eugene
Are you aware that vapor barrier clothes fit snugly with the idea that they will prevent perspiration because the air next the skin is already at 100% humidity? If you are in a loose non-breathable bag you will still perspire, only it will condense and run down the liner and soak you and whatever you are wearing in your own sweat. Be prepared to get out of your bag in about an hour and removing the wet liner and getting back into your bag wearing wet sleeping garments. You will then probably be colder than if you hadn't tried the liner. I'm just suggesting that you take a spare set of long underwear and maybe putting your jacket over you and maybe trying it some place safer than several miles from your vehicle for the first try. YMMV
Jim