Make them an offer and they might update their priorities. Sometimes it works. In my byz campaign, the Venetians were reduced to their capital only and I had a huge army outside, led by no other than Emperor Whats-his-name-again the merciless. Still, they considered a truce to be very demanding. Nevertheless, I made the offer and they refused and were pretty angry. However their priorities were updated to "peace", and when I asked them to become vassals, the venetians considered it a balanced deal and agreed. Go figure.
I was playing LTC so I dont really know if it works in vanilla, but it might be worth a try.