what is vacuum pressure
vacuum pressure = absolute pressure inside the vessel - absolute pressure outside the vessel, when the outside is greater than the inside. Vacuum pressure is the amount of pressure that inside the container is less than outside. Since the damage is done by a difference of pressure, the vacuum pressure means that the force is pointing to inside the container. But because the hard vacuum at 0 psi, and the outside is 14.7 psi, the vacuum can not be greater than 14.7 psi.