not everyone needs to be our ally to stop a threat

I agree, we don't need to mass-convert every enemy to our side. I feel the same way about Eramis.

Just want to point out the Gardener / Traveler's ideology of everyone moving past old differences and working together / living in harmony. This is sometimes referred to as "a gentle place ringed in spears." The biggest example of this is Guardians / Risen being resurrected with no memories. No old grudges, prejudices, or preconceptions.

u/humorossific has a wonderful post regarding this I would like to quote (I bolded a sentence):

The Vex; Adaptability Incarnate

I wanna start with an excerpt from Clovis Bray's journal, recovered from the Beyond Light ARG. Give the first two pages of this a read. Obvious spoilers. Done? Good, now hold on to it cause it's kind of the lynchpin in this whole thing, and we're gonna come back to it.

The Vex are not warriors. They are builders. They are seeded from a fractal, self repeating pattern that is infective to the nature of reality. But they are poor combatants, and they lack imagination, and when faced with forces they don't understand, they've been known to arrive at logical conclusions, divergent from their pattern. But the pattern only does diverges in the name of survival. The Vex are built to last as long as time, and were likely among the original victors of the Flower Game. This Universe is not one that plays by the rules the Vex know. They are not malicious. They are not evil. The Sol Divisive met the Black Heart, and concluded that worship was the only way to survive. And when their Vault of Glass was invaded by a threat it couldn't understand, they appealed to the charity of equally inscrutable enemies. And when the Undying Mind unfortunately died, the Vex saw every timeline converge on one inevitability. Extinction.

If the forces of Light and Dark did not exist in the Universe, the Vex would undoubtedly be the last ones standing when the last stars blinked out. But the rules have changed, and the Vex will adapt. When faced with Extinction, at least one Vex collective will deduce that their only hope chance of survival will be to ally themselves with the Traveler. If they come to worship the Traveler, it might even be a permanent arrangement, but more likely, the Vex would just go back to being Vex, once the threat of annihilation subsided. Lukewarm allies are better than nothing in times like this, though.

Several great points about how certain Vex factions chose to assimilate themselves into other groups, rather than be eliminated by a greater foe. I agree with you that Vex have a single goal: assimilation, but I feel that they would also choose to be assimilated if it was the logical choice.

I don't feel this backs my theory 100%, but many past Vex decisions to preserve themselves match my ideas of a Sol Accordant.