I was in an alliance with the Zhou and at war with the Franks.

Midway through the war, the Zhou became a vassal of the Franks.

This ended my alliance and set me at war with the Zhou.

However, since neither of us have positive war support towards each other, I am unable to attack any Zhou troops, and they don't seem to be able to attack me either.

I am even able to walk through their only city freely.  I can provide a save file if needed.