The Chinese, if I remember, often had to send their army down to Vietnam to collect their tribute (Vietnam didn't consider itself a vassal, China considered them so).
Perhaps a mechanic forcing the central realm to move nobles (or just the ruler or an ambassador) to actually collect the funds would be interesting.
This is not new mechanics at all. Currently, you are able to send down armies to punish realms who won't do what you tell them to, and for them to give you monetary tribute, a noble must move so that a gold transfer can be made.
