YA Modern Fantasy girl travels to other world

This book is older, I read it probably about ten years ago. The main character is a teenage girl who moves to this manor in New York City, and after moving there she starts being able to travel to this other world. I'm pretty sure that it happened in her sleep. When she first arrives in this other world, she finds herself in this cave system, where a group of people find her, and it is later discovered that she is the key to saving the people of this world. She has this magic necklace that I think was the key to her being about to travel to this other world. At one point it is revealed that the people that she is now living with are actually vampires. Another point in the book the manor that she is living in gets attacked by monsters. Later on they fly her out to this cabin in the middle of nowhere in some mountains in order to protect her. Also I think someone almost dies on the plane, but that could be wrong.