In the book And Then There Were None,

What has really happened to Vera in her room?

Vera killed herself in her room. After killing Lombard, she went into her room and there was a rope hanging from the ceiling and a chair under it. She stood on the chair and hanged herself. She was the only one in the island left after all the other guests on the island were murdered/died. She wasn’t murdered but she killed herself of her own volition because she felt guilty of the death of Cyril, the little boy who she once babysat.