OK, so I went into worldbuilder and made myself a vassal of Ghengis Khan before a single civ had settled. I met Khan during the first turn, even though he was half the world away from me. I was pretty isolated from every other civ. Then I met each civ that he met as if we were on the same team. He was able to walk on my land years before either of us had discovered writing. I didn't discover where he lived until well after that, so didn't even try.
As soon as I got horses, he demanded it from me, which was fine, But then he demanded those same horses every single turn. He didn't seem to be able to keep them like a regular trade. Later when I found iron, he demanded that instead, not in addition to the horses. I thought maybe he had horses of his own, so checked in worldbuilder, but he did not. Maybe he can only ask for one resource?
As far as resources went, maybe it was a coincidence, but when I went to the trading screen, none of my or any other civ's resources appeared. Only the techs showed up -- except when I was talking to Khan. And then, only my resources appeared. I couldn't even offer to trade money for one of his.
But what I found interesting was the affection he seemed to feel towards me. Maybe it was because I could cancel my vassal at any time due to my size, but he was just downright nice to me. His status was officially cautious, but he was gifting me techs and resources even though I was right next to him at the top of the score.