Riknap, you might want to look at mercenary cost balancing again. In my Palmyra game, non-vassalized mercenary monthly costs were so low that I was able to keep them raised between wars as a standing army. In my current Isauria game, the cost of keeping the vassalized Isaurian Guard is so high that I had to dismiss them in the middle of a war, before I'd ever met the enemy's main force. And I'd gone into the war with what I'd thought was a decent bankroll.
that's pretty much the intent really:
non-vassalized mercenaries are expensive upfront but cheap to maintain, though they barely manpower while active - they are replenished quickly while inactive though (a sort of reorganization/recruitment phase that happens while not flying a lord's banner is how I view it really)
vassalized mercenaries are a third cheaper (relative to same-composition/number companies) upfront, but thrice more expensive to maintain. they also replenish at the same rate as non-vassalized mercenaries, ie. almost not at all.
I guess part of the issue that throws the figures askew is that vassalized mercenaries tend to be expensive-type units (HI and HC and/or HA mostly), while regular small-company mercenaries are a random mix.
Still, I'll try looking into closing the gap between the numbers again to make the difference not that drastic, while still keeping the distinction between the two noteable. I was trying to avoid making vassal mercenaries the instant-win trump card they used to be, as previously vassal mercs were both cheaper to hire and maintain, which made realms that happened to have them guaranteed blobbers (so long as the finances are too messed up).