Okay, do you need to be friendly to request vassel state.
There's only two ways that another civ can become your vassal, other than colonies. First, they volunteer to become one and you either accept or refuse. Second, you have to beat them badly in a war and they agree to Capitulate to you and become your vassal. Other than that you can't just ask them to become your vassal, the option will always be redded out.