I don't think it's a good idea to turn companions into vassals IMHO. Especially after training and leveling them up. If you are desperate because you have more fiefs or towns than you can manage and are unable to recruit lords from other factions usually means that you did not invest enough time in the early game to increase your honor, renown and some RTR. I spend close to a year being a freelance and leveling up my companions. This is usually how my game looks like if my goal is start my own faction: (I can only account for the DIPLOMACY MODULE)
Early Game (March - December)
1. I start off solo and tackle as many sea raiders, mountain bandits, deserters, etc. as I can find.
2. I sell off the loot and try to scrape up enough money to start industries in every town. This guarantees me a large and steady income to support my mid to late game army and garrisons.
3. I hire as many companions as I can. Sea raiders loot include armor and weapons that I can equip my companions with for free.
4. I also talk to bards in taverns to learn about eligible maidens of a faction and to learn poems to use during courtship
5. I compete in as many tournaments as possible and attend feasts to meet all of the lords of a faction. This way, I can learn what their personalities are for when I start recruiting in the Mid to Late game.
6. I select lords that are "good-natured" or "upstanding" and try to curry favor with them by asking for and completing quests.
7. I always check if any lords are imprisoned in whatever town I go to. If the lord's health is at least 25%, then I will try to find out if they have any relatives and where those relatives are. I then ask those relatives (usually the imprisoned lord's wife or father) if there is anything I can do for them. If their husband/son's health is at least 25%, they will give me a "rescue" mission. Completing that mission will yield positive relations along with a little bit of gold with the mission giver and positive relationship points with the rescued lord. It will also yield a little bit of honor.
8. Lords will some times give a "hunt down a murderer" quest. Completing the quest results in positive relationship points with giver. Refusing the reward money will also yield a few honor points.
9. I also look for farmers in taverns. They will give a "bandit infestation" quest. Completing the quest and refusing payment will also yield honor points.
10. The point is to spend almost all of your time increasing your honor and renown in the early and mid game. High honor will automatically improve your relations with ALL "good-natured" and "upstanding" lords regardless of whether you have met them or have yet to meet them. This is important because recruiting and retaining "good-natured" and "upstanding" lords is key to a smooth running and trouble free kingdom in the mid to late game.
11. If I am a mercenary with a faction that is at war with another faction, I earn RTR points when peace breaks out. So I might have to renew my contract until that happens. After the contract expires and I have received my RTR points, I sign up with a different faction that is at war. Why? Because I try to keep the balance of power even until I start my own kingdom. This keeps one faction from getting too powerful and destroying my kingdom during it's infancy.
End of Early Game/Start of Mid-Game: (December onwards)
1. By now I have all of my companions leveled up to around lvl 20+ with decent equipment and can survive being outnumbered 5 to 1 in battles.
2. I study the map and decide on what town I want to conquer to start my kingdom. Then I wait until the target town's faction is at war with another faction. I then sign up as a mercenary with the opposing faction and help weaken the target faction. This is IMPORTANT: I save my game often. If my temporary faction's marshall decides to conquer my target (kingdom) town, I bail out of the game AFTER I NOTE THE DATE THAT THE MARSHALL STARTS MARCHING TOWARDS MY TARGET TOWN. I re-load and capture one of the enemy's castles that is furthest away from my intended town. MIGHT HAVE TO ENABLE CHEAT MODE AND TELEPORT TO SAVE PRECIOUS TIME. I then garrison the captured castle with no more than 10 troops. The low garrison strength ensures that the enemy faction will not hesitate to try and recapture it as soon as possible. This forces my temporary faction's marshall to keep riding back to relieve any sieges and keeps him from laying siege to my "target" town. Rinse and repeat until my contract expires. Once my contract expires, I recruit enough troops to capture my "target" town and start my own kingdom. I will now be at war with a weakened faction that is getting hammered by another faction (the faction that I left when my mercenary contract expired). From my experience, the faction that I captured the town from will offer a truce with me shortly. I then earn RTR points.
3. I then select my future bride base on notes that I made after talking to the bards found in taverns regarding the eligible maiden's personality. I always choose "adventurous" because I will always have my wife join my party as a companion. She can then be trained and leveled up just like any other companion and she draws no wages regardless of her level. Further, I only choose a maiden that has a mother. Because I am able to appoint my mother-in-law as my minister. This saves me from using a valuable companion in that role. I invariable choose Jarl Harald's daughter because he is "good-natured" and she has a mother. Courtship is relatively easy once I get Jarl Harald's approval and I get RTR points after the wedding.
4. With my relationship with "good-natured" or "upstanding" lords in the mid to high 60's and above, I constantly check my reports tab>KNOWN LORDS BY RELATIONS. This shows that lord's relationship with their liege. If the relation is less than 10, then your chances of recruiting that lord increases. I am always patient and only look to recruit lords AFTER I SLOWLY expand my kingdom beyond my first town.