When your paid-for vehicle is titled in someone else's name and there is no way to prove how much each party paid can the title holder take the car away or charge you with theft?

If your name is not on the car, it is owned by and property of the person on the vehicle registration info. It is not relevant who is on the lending agreement, the person(s) whose name(s) are on the title to the vehicle and the wording of said title determines ownership. The title holder(s) have legal rights to a vehicle and if it is taken without their permission they can indeed file charges. Whether or not said charge can be "vehicle grand theft" depends upon the state law.