The balloon is made to be elastic, so it will simply expand until it can't, then it will explode. You can't compare the resistance of a rubber balloon to that of a rocket. A rocket is not made to be elastic so it will keep the pressure inside.
But this is just the point.
The gases will expand inside that rocket because they want out and there is no pressure against that rocket to arrest that, so they will keep expanding until the rocket is breached.
But let's assume that the rocket is strong enough to hold these expanding gases inside it. It can do so forever if it's strong enough but like a patient cat waiting for an injured bird to drop out of the tree, the vacuum will be ready to swallow up any breach that is made to the rocket..................meaning..........
The minute the valve is opened for propulsion, the vacuum of space has it in it's mouth until the rocket is empty.
End result = no propulsion.
Again, you can't imagine vacuum as nothingness can't you?
If the engine is turned on, the expansion of the gases will cause propulsion, if they close it, the expansion stops, easy as that. Is not like the pressure inside the rocket is increasing constantly like if you keep shaking a soda. There is no reason for things inside to try to expand if they are under pressure (and is the lack of pressure what makes gasses and water expand in space), so they don't.
Look, water is always causing causing pressure in the pipe system, it has a higher pressure than the atmospheric, that is the reason if you open the tap, water will go out, but if you close it, it will stop going out. That has never been a problem.