When we were thrown from the tower by gaul the vex recovered our body and is keeping it alive and running a false reality in our minds(i.e. the matrix) to see if and how we would go about defeating the red legion. If we can defeat the cabal then we are of use to them if not we become waste. This could explain why modes have no selection and everything is limited. Reduced gear and limited abilities is worst case scenario. If we prevail in this environment then the vex know we are a viable option to defeat the red legion. I feel out of any of the enemies we have the vex would most likely become an ally. We have seen all their secrets and their main goal is to aquire knowledge. Why wouldnt they ally or become friendly with the one thing they dont understand. The friendly Harpy story mission on nessus would help guardians actually see a vex as a friend. No other enemy group has ever been a friendly that i can recall.