Not 100% sure but I'm pretty sure there is a drop you can get from the Hard VoG raid similar to an Exotic Shard that allows you to upgrade the Vex. Look into it, it's called... "Vex Raid Rare" here's where I found it.. http://db.planetdestiny.com/items/view/584130857
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A lot of stuff on that site was datamined and doesn't exist yet. If anyone had gotten that from the HM VoG there would have been multiple threads on it. Maybe in the future though.