The Vex won’t attack us unless victory is assured, and seeing as they can’t simulate Light, they cannot know our full capabilities
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Well they can simulate light, but it is unstable. For example, take the very first curse of osiris cut scene. They can simulate it, but it is very unpredictable in what it is going to do. Here is a link to a My Name is Byf video. Skip forward to about 28 minutes.