World Change - The Earthbound Trilogy, #1

Clare gets transported from present day Florence to an alternate world, one that looks like fourteenth century Italy. The only way back is through a portal half a world away, plus the local High priestess wants her dead, and the only one she can trust is a vampire called Trevi. He is the only one who exits in this world, and she knows more about what he is than he does. He is hard to resist, especially when he saves her life.
Trevi can take her home but he is keeping one big secret. She will have to be like him to survive the journey through the portal back to her world making a myth a reality.