I got my Sagira's Shell and as shown by the attached image file it shows I completed all 10 Lost Prophecy quests but I only have 9 weapons the Sword is missing and no new Quests from Vance
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You can only complete each verse once. So if you’ve already completed it, Vance won’t give it to you. If the sword is not in your vault, postmaster, or inventory, then it falls into the lost/deleted items category & cannot be replaced. However, you can get a duplicate (subject to RNG being in your favor) by ranking up Vance, and I also believe it can be randomly earned from the Flashpoint milestone and/or in Cayde’s treasure chests.