In the new Rise of Iron reveal the Fallen use a Tech virus to mutate and became more tech based organisms. If they keep mutating further I believe they will eventually become the vex.
Dregs = goblins
Vandals = hobgoblins
Captains = Minotaur
Shanks = harpys
Servitors = cyclops
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Nope. The vex has always been. Time means nothing to them. No beginning..no end..lol