You say that the place is vacant, but the person is living there...which is it? If it is vacant, then I think you are in a better position, but either way you should get a VERY good real estate lawyer to help as quickly as possible (before someone moves in).
Title insurance is not relevant here because title is not at issue, possession is.
If you need a reference for a lawyer, I have a great one that I use myself as well as recommending him to my clients. Contact me if you would like his name.