The Vault of Glass is still dropping the older versions of Exotic weapons. I hope that's an oversight in code, elsewise the incentive to continue fighting Atheon is boiled down to Ascendant material, which you can obtain on your own, without pulling together a group.
I picked up The Ice Breaker, and it was capped at 300 damage, after defeating Atheon on Hard. Please fix it at your earliest convenience.
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There is no reason for the Vault to drop upgraded exotics. Max level is 30, therefore drops adhere to the 300 ATK model. If you want to upgrade it, just hold onto it until Xur shows up with it in his inventory. The only places with good odds of getting upgraded exotics are the Nightfall, Crucible, and Crota's End. Only two of which actually benefit from having the upgraded version available to the player.