I think it's the peasant + loot perk that makes vladia the strongest faction. Basically you can take peasant a couple of times and make enough gold to take sergent for the rest of the game.
By that logic, Khuzait would be top tier, since their looters have a shield in their base kit, and I think the Eastern Sickle is the best one-handed cut weapon a peasant can have in Bannerlord
Honestly though, I think the Sergeant could use some slight number changes here and there, and I just want the Sharpshooter to be more vulnerable in melee to make up for being the strongest ranged unit.
As for the Vlandian Knight, there is one buff that I personally wanna see. The shield perk is honestly kinda useless since the Cavalry Sword is so painfully slow that you're an even worse melee unit than the Imperial Recruit if you ever lose your horse. I think it'd be worth it to replace the stronger shield perk with the following:
Mounted Infantry
Downgrades your lance to a Vlandian Long Spear. Shorter and can't be couched
Your horse has 3 less points of armor (stacks with Flanker)
Upgrades your Cavalry Kite Shield to the Voulgier's Kite Shield. Slightly more durable, much greater coverage
Upgrades your Vlandian Cavalry Sword to a Vlandian Axe. Much faster, deals bonus damage to shields.