Yes, but seeing as I was clearly talking about the Forerunners, they're irrelevant. At best the modern Haloverse would be in a support role. Anyway, the Flood don't get infected. They recognize a potential threat and adapt to it. Any form of biological matter is convertible, or consumable, that's sort of their super power, so there wouldn't be an exchange of infected, there would just be more food for the Flood. And you ignore the logistics of getting to the Halo rings. You would first have to find them, and then get through their defenses, not the least of which is their 1 light year ranged defensive weaponry. The fleets defending these installations would not permit any entry, nor would the Ring itself, which would throw itself into a fractal dimension, preventing any and all forms of external manipulation (this is how it defends itself from its own pulse). Even if you were to get past this for one Ring, all Halo has to do is shoot it with another Halo Ring and obliterate any enemy force attacking it. And there is no dodging it; space and time have no meaning to the Halo pulse, according to Silentium. Really the Vong is just about the worst thing to bring up against Halo, what with the Flood foaming at the mouth for people and technology like them. Even if they got past every defense one of the Halos has, the Sentinels need only to deliberately breach Flood containment facilities and let them loose on the Vong's bio-exclusive army. It would be disastrous for them.