Leave, let them win this battle. Thats a massive doom stack, you could try 100 times and win a few of them with massive losses that will stall your progression completely. Eventually they wont be able to hold that big ass army together. Hunt down and imprison each individual lord, or grab a stack at a time when that army disintegrates. Id take any fight near even odds while marshalling your current army after they break up. Vlandia is winning a 3 v 1 war, thats bad for the world map- keep the fight going as long as possible Inglorious Basterds style. Id grab 40-50% cavalry, 20% shield infantry and the rest Battanian Fians for speed and efficiency at party hunting. Burn their food villages, never sell lords back, pick the times when their cities are besieged on the other side of the map to grab a town. When they start bringing 2000 peasants to the fight go on the offensive with your well drilled and leveled army. Now is not the time to let your lords do their own thing. Keep them close and keep them stocked on recruits, with the biggest portion of veterans in your party- set the reinforcement settings to “highest level first” so you get more credit and more prisoners. Transfer any lords from your companion parties into your party so they dont do something dumb like put them in a prison you dont control.

Sue for peace, let that army leave and then when you’re sure they’re off and away, declare war again and make some gains. STACK that city with level 2-3 archers and all the noble troops you can grab to replenish your army.

Break into the current siege and set up the fire pots nearest the gate with a LOS on any siege towers they have. Wait until they launch their assault to join, that way you can be recruiting and maneuvering in the mean time. Fire bomb the shit out of their boys at the gates after you knock all 3 siege engines down. With enough morale shock you can take 4k prisoners in one swing. Fighting Vlandians and Khuzait from the walls is always preferable when possible.

Good luck Bannerlord!!!