I've made multiple sovereigns with the warlord trait and managed to make one of them plus Verga a vassal in one of my games. I then had my own  warlord Sov and a custom warlord sov and Verga as vassals. The end result was that my army was paying me for the right to fight and die for me. With a warlord sov and a vassal with warlord, all unit wages are 0. I'm assuming with a third warlord, unit wage goes into the negative, so the units end up paying to be in the army.

I'm sure a situation every fictional and non-fictional warlord would love to be in, but probably not working as intended.