I think the traveller created the vex in a different timeline to combat the Worms first attempt at universal domination. They succeded but continued to wipe out other sentient beings so were sealed away in another dimension/timeline until Savathun tricked Crota into freeing them.
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From what I've read from the calcified fragments, Crota inadvertently created the Vex from purposely wounding himself, experimenting with oversoul theorem with his sisters. The first 'mind' that actually started studying how the Hive did their business and tried to emulate it was Quria, who learned the sword logic and tried to challenge some of the more powerful Hive leaders... including Oryx himself... now, the details aren't clear if Quria eventually got free, or if it's still in Savathun's possession... I look forward to fighting it should it emerge in a future DLC