I recommend you read Solution ID: S-056539 from the SOLIDWORKS Knowledge Base. It is entitled "Recommendations for simulation of tank filling and evacuation".
I guess I should mention that I wasn't talking about gas cylinders. The vacuum pressure is typically less than -5psig which is not enough to initiate sonic flow, usually the mach number is less than 0.8. I think it's reasonable to neglect compressibility effects up to that point. I was just curious what everybody thinks about this type of flow, in general terms.