So does the stat of your companion matter when they become vassals? I believe combat skills will but how about skills like surgery, path finding, and especially leadership (would this determine how many units they have?).
I was wondering that too. From what I've seen they tend to start out with army limits around 40-50, which is pretty bad. The renown companions join with appears to range from 240-270 depending on their background. My guess is that their stats/skill do matter, as the few companions that I did make vassals all had low leadership scores. I suppose that's obvious though, since the game doesn't let you adjust their leadership when you level them up.
I made Lezalit a vassal and he's slowly increasing his party size cap (up to about 70 now) but he won't improve his skills and he's leveled up. He has a little + by his name but he refuses to do anything about it! What a goon.
in my experience recruited AI lords are much better choice then your companions since they build up their army much faster. In one game I made Matheld my first vassal when she's lvl25, and despite owning the most fiefs (1 castle and 3 villages) she only raises an army of about 80 after about 300 days. On the other hands, the three AI lords I recruits much later, who only have 1 castle and 1 village manage to raise a 130-180 army each. Not to mention if your companion's army gets smashed it takes them like forerver to raise their army back.
Unless you're doing it for RP purposes, or need an emerncy side kick to help you defend a newfounded kingdom (that's why I made Matheld a vassal), don't use your companion as lord.
"Why do you submit to the flesh when death is bred in your bones?" Jon Irenicus.