I found a 6th chest in VOG I have collected it once but like the 5th chest you must do something specific to make it spawn and we dont know what we did. EDIT - I will post a video of the 6th chest location by I have yet to find out what makes it spawn so if you guys find out please post
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When you first walk into the atheon fight the door is open and doesn't close till everyone dies. If it can be done without wiping then maybe you can leave and collect the chest at the broken vex portal. Otherwise maybe killing a gorgon and having the two chest doors in the laybrenth close allowing another to spawn.