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The vex, no matter when you have gotten it, was patched to have a final damage rating of 323. Pre-patch vex's like mine were changed in value, and post patch drops are at the same values for attack ratings. So to answer your question simply if you get a vex now it will be upgraded yes (but you still have to level it like any other piece of gear. It doesn't drop fully maxed out with experience sadly)
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There isnt a upgraded Mythoclast yet. Defeating Atheon will only grant you the lvl 20 version.
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