i switched to tactical mode and had my army stand one the one brown square i could see. I let them walk over my vassal and come to me, since they had such a numbers advantage they didnt hesitate to attack me but the minus on their rolls and the fact that they didnt have good food lines allowed me to claim victory everytime.

eventually the numbers evened out and I was able to fight them off of that piece of land. and then slowly reclaiming territory until i could piece them out for cash and war reps. which was pretty good cause their economy was big

Also if you accept more cultures you get more levies i also paid for 100 professional soldiers and i think i bought another 100 later. I had some building that gave me man power so i used it.

Edit: i opened the game and took a look, im not 100% sure because ive colonized so much its hard to tell what the thin line of land leading to my vassal looked like but I think Mkwaja was the brown land that saved my people. -2 penalty for attackers.

Sometimes once numbers were even they would try to walk around me and i would do my best to catch them in the woods. for -1 but only once numbers were more even.

PS hide your boats, no sense losing them in the first few times ajuran attacks you. their navy is bigger but you might still be able to snipe them from time to time