Why do vassals only tribute infrequently during war and not at all otherwise?

[Birken]

You get a percentage of their tax income and you have the power to raise levies in their provinces.

 

[Birken]

You're not supposed to have levies raised unless you're using them; disbanding levies in peace time will drastically reduce the maintenance cost of your armies (since you'll only be paying for your retinue + mercs).

Additionally, I believe vassals contribute a drastically reduced (maybe zero?) percentage of their income if they aren't making a whole lot to begin with. It's to encourage you to spread your kokujin titles out instead of giving only one to each vassal.

Yes, there's a basic deduction before a percentage is paid. Small vassals will usually not contribute anything since they usually upgrade their one or two provinces.