I would advise against any type of pad/vapor pad in a bsmt situation. Its a given fact that all bsmts below grade will get wet. By placing a pad/vapor pad under the carpet will contribute to larger cost in restoring.
If your inclined to pad your basment drylock the cement. Leave the pad with the vapor barrior for a pet situation. If you have moisture under the vapor barrior wouldnt you want to know about it? Not only will it keep water from soaking into the pad it will keep water from coming into the carpet.
IMHO Dryloc and carpet glue down.