From a biological standpoint, I do not see how the Flood could survive in vacuum. If the infection forms are easy enough for the Master Chief to pop with his hand, there's no way they wouldn't explode in vacuum. Same thing goes for the carrier forms. Now, the combat forms are a little hardier than the infection and carrier forms. They can take a bit of damage. I think it would be presumptuous to say that they could not survive in vacuum at all, but I am sure they would lose a lot of their body to the effect of rapid decompression. I think a few would survive out of luck, as long as the central nervous system of the host isn't damaged by the decompression.

Consider this: the only reason there was Flood left on the Halos is because they were specially "preserved" by the Forerunner. In fact 343GS says he is happy to see that some survived to reproduce, which suggests to me that this was in doubt, even with the preservation.

Flood must die in vacuum, and without food, or else why the hell fire the Halos at all? If the infection forms are going to be hanging around waiting to infect the first new life that arises, then why do it at all?

Think in larger terms. We think it's great to break the sound barrier. Earthlings of 2552 think it's great to break the light barrier. What would we think is great in a million years? In the realm of SciFi, you only need imagination. If the Flood infected an Uber-Advanced species in another galaxy that had Supersassy Spank Drives that provided galaxy-to-galaxy travel, you'd have your answer.

Obviously the Flood took control of Atlantis and dialed the gate to our galaxy.