There are always fissures in vacuum chambers, and air is constantly getting in, which is why there is need for constant pumping.
Yet even with these fissures, the air outside doesn't "immediately" fill the chamber, since vacuum, as I said, isn't a magic "pop and it's gone" thing. It's just a pressure difference causing air outside to go in, but not at magical breakneck speeds to fill the chamber immediately. Even if you open a larger valve, air doesn't "immediately" fill the chamber, it takes quite a few seconds usually.
So there you go, gas doesn't instantly cease to exist when in a vacuum.
Consider the pressure differences again: vacuum to atmospheric pressure is only 15 psi. A car tyre already has twice that. And then imagine the pressure of the exhaust of a rocket. 15 psi makes almost no difference for it.