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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this is only because VoG still drops old loot
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Ya, every exotic droppped from the vault comes with the 300 attack rating, but because the vex is unique and they wanted people to use it for the new raid, it had an above average attack rating implemented. All exotics can be buffed to 331 attack with xur, but the vex is the only one that comes with a high attack stock.
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So you have no idea if it can go higher or not until xur sells its upgrade
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IF he sells an upgrade for it. Correct. Bungie has not commented on whether or not the vex specifically will be upgradable via xur, but being that it has a lower attack value than upgraded exotics I think there is a strong possibility that it will.