Might be that The Nine and the Entity are on opposite sides, and are older than the time, but in this era they are just betting who can create stronger followers. I strongly believe in something along the lines of Traveler being created by the Nine (Cabal and Eliksni are calling it The Machine, maybe with a very good reason), then being held captive by whoever is on the Entity's side, Entity itself or the Hive, until it escaped and eventualy stumbled upon us.
I'm more curios where exactly Vex fit in. There was a story going before Shadowkeep dropped (not sure tbh) about Black Garden, and it was hinted that Vex were Darknes' version of Guardians. Don't remember the exact words, but it was sometng like "Darkness asked a question, Vex were the answer." Now we don't hear much about the Vex.