According to Praeydth, the Vex had forseen their fate billions of years before the Taken War. It's possible that the Vex were willing to take the blow on the off chance that the Guardian stopped Oryx and saved them.
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Which would also be a plot hole because the very existance of the precursors and descendents
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It always interested me how the Precursor (which should be the old ones) look super chrome and glow like a TRON toy, and the Descendant (you'd think newer chronologically) look low tech, bulkier, and rusty. Like the "first" vex are actually from the future, and "future" vex are ancient.