So who would win in an all out battle? It takes place in our solar system and they have equal numbers and resources. Also, the Gravemind is fully developed.
Both also have access to the fallen, cabal, hive, etc. So it's an all out battle to infect/posses as many victims as possible, and defeat the opposing force.
The Flood can also (possibly)infect the Vex as their Mind Cores are biological.
Some quotes:
[i]Every end crawls from the same pit, rising from the schism to swallow matter, Light, and life.[/i] -Osiris on Oryx.
[i]The Flood must not escape this installation, they would consume [b]all[/b].[/i] - 343 Guilty Spark, Installation 04.
Flood info for non Halo players:
https://m.youtube.com/watch?v=4vPzIOfblwQ
Info on the Taken:
https://m.youtube.com/watch?v=xKiqZAJAUvA
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Nothing can escape the flood.