The second law of thermodynamics states "that the entropy of an isolated system always increases or remains constant", see
http://en.wikipedia.org/wiki/Entropy. This means that a pressure difference will always tend to equilibrate if no barrier or pump prevents this. The pressure of a pressurized vessel will decrease if the valve is opened to the atmosphere. This is achieved by fluid flowing.
The same is valid for a vacuum vessel, but inversely. If the valve is opened the fluid flows from the environment into the vessel, raising the internal pressure up to that of the atmosphere.
It has to be noted, that inside pressures can be many times higher than that of the atmosphere (1 bar), but that no “greater” negative pressure is possible than -1 bar (referred to the atmosphere of 1 bar) = 0 bar = absolute vacuum. Nobody can release a pressure of 0 bar as no (negative) pressure exists below the vacuum, which has no pressure per definition. Otherwise spoken, nobody can release a gas, not contained in the vessel, to the environment. And “vacuum” means “no gas within a specified volume”.
I hope that everybody can understand these principles of physics.