I don't know, the Halo's defenses kicking in at minimum safe distance of 1 light year or any of the plethora of ships defending the Ring? The Precursors and the Flood are also trans-universal, so that the Vong live in another galaxy is moot. But you can't just say that the Vong would do something on the basis of "lol why not?" There are factors to consider as well, such as the Halo's being one of the most heavily defended installations in Forerunner space. What exactly can the Zombies do? And the Forerunners would eat the Mandalorian ground forces for breakfast. They would in fact win against any Star Wars ground force because Star Wars still has a modern military mindset. The Forerunners just tear apart the surface of a planet from the air, they only go into traditional ground engagements to root out specific targets. But to compare, Forerunner civilians wore armor 4 times as powerful as MJOLNIR by the end of the war, the Flood still breached them, because infection forms are only one way to get killed by them. Simply touching or breathing the same air will infect you without environment suits. Hell, once Keyminds are formed, simply being in the same star system can start causing feelings of unease/fear in the back of your mind.