At the jumping part after the gorgon maze there are two caves at the left side very far down.. at the beginnig top right is an invisible wall. The lost portal in the middle is reachable, but if you can activate it, i dont know how. Maybe theres another way from the gatekeeper side after you started fighting him or defeat him. Gatekeeper side far down i found nothing except "guardian down" and far up anything either. So maybe IF theres a hidden chest nobody found before, they spawn in that cave spots or on the other side of the portal... maybe the portal links to another part of the VoG .. new content. Like the gorgon grimoire card says "the VoG isnt complete explored yet" (something like this) maybe the chest doesnt exist and its a rumor from some bungie trolls. Maybe it is somewhere else (like the hidden path to the confluxes at the beginnig) maybe an special to-do list you have to complete to let it spawn like the templar bonus chest.. we all dont know. I hope they will extend the VoG with more bosses in it. Best raid so far.. (Sry for bad grammar - maybe - my mother tongue isnt english)^^