It's not done because you will find a vapor barrier gets things wet unless your typical night temps are well below freezing. Some people find condensation just using a pad. Your body is going to be a source of condensation. This is why folks use vapor barrier liners in subzero temps - they want to keep the moisture from freezing inside the insulation of their sleeping bags, which unless kept under control will result in loss of loft.
I'd use regular nylon for an undercover for 3 season backpacking and keep the overcover separate, so you can leave it at home until you really need it.