The Seraphim Vault could be reached in the beta by visiting the Forgotten Shore area and using a Sparrow to jump over an invisible wall, letting you reach the water without dying. From there you could access other areas of the map and find the vault. Here's a tutorial (via Reddit) showing how to get there,
And here's a video of the vault itself:
The current theory seems to be that Seraphim is some kind of AI, and this post from over a year ago suggests that finding the vault is potentially a big deal:
"Some time later we meet a crazy scientist living in the middle of nowhere, when we tell him that the seraphim are still alive, he says thats impossible in a very dramatic way. Then we're told that if we find them we will "BECOME A LEGEND" in such an overly dramatic way I start to think the writer is trying to ham it up on purpose, maybe hoping I wouldn't notice the copy-paste of the borderlands plot that just happened. We now have an over arching quest to find the seraphim's vault/restingplace/mcguffin, whatever it is, this story is sparse on the details."
It's worth noting that 'legend' has a specific meaning in Destiny with Bungie story lead Joe Staten calling it 'a sequence of events and missions that make up a story in your world'. In the context of the above playthrough it's the first real mission encountered suggesting this could be the 'legend' that takes the player away from Earth and into space.
The reason I think that is because this is not the only vault. On Mars there's Charlemagne's Vault with Charlemagne themselves described as "one of the great Warminds of the Golden Age—a vast machine intelligence built by the ancient Powers of Mars." Charlemagne's Vault was first mentioned at Bungie's GDC 2013 panel where it was mentioned you could get Thorn, a high level hand cannon for completing a quest in the area. Another location on Mars called the Dust Palace apparently contains the "lost treasures of Charlemagne" which have been described as "ancient powers of Mars". So it could be that these vaults are large plot nodes between sections that unlock new tech/weapons to progress your character.
Below is alleged concept art of the Mars vault. Anyone got any other info or ideas they want to add?