I personally think it's a little bit bullshit that vassalage existed because administrating so much land was just that hard. I think the reason vassals existed is not most of all because a king couldn't find the people to do administration for him (literacy), but because he needed to reward his allies / generals and lieutenants (and they needed to reward their captains etc.). And the way of doing that is by granting someone land and most importantly the value produced by the people on it. It was a path dependent development not a universal functional necessity. You're a warlord, you've been super successful, you want to make that glory and power endure, so you extract REGULAR loot/rents from the people you've pillaged/extracted submission from. Ok. But you've got to give the people you've gone to war with something. Loot? Sure. Loot is also land. These buds want to settle down and have THEIR power endure and have THEIR children inherit also. You get vassalage instead of royal/imperial administrators. Because the only way to have a secure retirement is to extract rents. You owe them something, they have troops loyal to them. And if you install royal/imperial administrators - those want their children to inherit their power also: vassalage. Depends on your level of control whether you can stop this from happening. Depends on cultural expectations whether this happens at all. I think.
