Don't depend solely on the integrity of the tube. If the ground shifts or settles, which ground often does, the tube could crack or break, letting water in, or worse. This happens frequently to buried PVC pipes, which is why waterproof, direct bury electrical cable must still be used inside underground PVC pipes. It's quite common to open up buried PVC pipe and find it filled with water. Waterproof bags inside the vault tube would be a good backup and the cosmoline idea was a good one.