Formations make a world of difference, unless your army is completely composed of Swadian Knights (as mine was when I was a useless noob). My favourite Vaegir tactic is to park my guys in a valley (so I don't have to worry about forgetting to tell my archers to fire at will- which is a bad thing to do) in this sort of composition:
I I I I I I I I
CCCCC
AAAAA
The enemy AI usually heads for the centre, and once they get nearish, I circle behind them with my cavalry. This usually breaks them up a bit, and quite often will draw their cavalry away from my foot troops. Once the main body of their army is engaged with my infantry, I swing my archers up behind them where they can shoot into the unshielded backs of the infantry to devastating effect. The cavalry is kept busy following me as we mop up any straggling archers etc. Once the first wave is pretty much crushed, I put my guys back into formation and await the 2nd wave. The third wave I let everyone charge- you gotta let loose sometimes.
The only problem is getting a balanced number of each class. I wish there was a mod you could choose who shows up for the battle, plus the reinforcement waves. It sucks when you get fourteen infantry, six archers and two cavalry.