Well the traveler thought the Vex were evil enough to stop them, and I'm pretty sure a Semi-god creature could tell the difference between friend or foe.
English
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If you're referring to the battle that the speaker "could tell us about", but didn't, I don't think that battle was against the vex. I think it was a hive invasion (which would explain how they got the shard of the traveler).
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When did the traveler stop them?
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When the ghost was talking about Mercury being taken over by the vex he then said "They would have done the same thing to earth if the Traveler hadn't stopped them"