I have found a way into the Seraphim Vault during the Promethean Code mission on Earth.
Reach the end of the mission and before you scan the console to end the mission. Return to the Seraphim Vault area and have a friend join you. They should spawn inside the area of the vault where you first meet Omnigul. You can then leave and rejoin your friend and you will spawn in the Warmind room as well.
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Did this on accident and noticed that one of the doors is actually scanable. Cayde says something about there is nothing behind this door but bad memories, the good stuff is below. Is this door scanable from the other side as well? Or did i somehow load into a new instance of the vault? There are also yellow blinking towers in here bow that i don't recall being here before.
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there is a death cornor that does this wihtout having to leave the session