I like this vassal thing, but the limit would be progressive.
For every region -or for distance to the capital- the efficiency of the taxes would go down. Is a lot more easy to get the taxes in a region neighbour with the capital than in a distant region, far, far away! 
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.
