One of the best ways to continuously make money, while being able to support a decent sized army is to invest fairly early on in businesses through guilds. Having a high trade is essential, but you can really roll in the dough if you invest in the right businesses in the right places. Vetluga is optimal for building a winery, since grapes are usually fairly cheap to buy there, etc. With this route you can build up a decent sized army, take over a wealthy town, and start your kingdom from there, no need to even become a vassal.