The Vault of Dreamers question

I didn't even understand the ending!
So who was the voice in her head?
And how did Rosie "leave"?

The ending was abrupt, and dark. I'm gathering that originally, the sequel wasn't a given. So it looks like the author went for a "stand-alone open ending." Something that could both stand on its own, and lead to more, if that should happen. Which, thankfully, it is, since not only is the ending abrupt, but there are a lot of developed yet unused characters, and more than a few questions left.

As for what happened at the end, I took it to mean she was going to go into a permanent coma, where she would escape their probes. I think the second voice was all her--you note that one of the side-effects experience by the alumni who killed themselves was hearing voices. Then there was the fact she doubled-down on that antidote pill, the one that Burnham stressed twice would “fry her brain.”

I would not be surprised if the next novel is narrated by a different character--maybe Linus, maybe Burnham--as they try to get her to come back to consciousness.