modeling the vacuum is done basically the same way as it as the pressures are acting in reality.
When creating a vacuum, initially both sides (internal and external) are under atmospheric pressure, then the vacuum is created by removing the air from inside, resulting in zero internal and atmospheric external pressure. The zero internal pressure does not need to be modeled explicitly.
I hope this explained the setup from @dheiny well enough. If there are any additional questions I am happy to help!