Byzantium, late 1454 here. I've taken Turkey and most of Greece, and I have Bosnia, Serbia and Romania* as vassals in the west, Georgia (in Imereti only) and Armenia (Van + Armenia) in the east. My Romanian and Georgian vassals border the Golden Horde, my Georgian and Armenian vassals border the Qara Koyunlu and my Armenian vassal borders the Timurids.
HOWEVER, despite the GH and QK truces expiring, they did not DOW. Furthermore, when I went to DOW them (as a test), I got the standard-issue "Nomads cannot be at war with their neighbors" sign. I know it's not because of the vassals, as Romania dragged me into the last war with the Golden Horde. Further, I can tell something's up because when I hover over the truces tab, it says "Truce [with GH] expires on April durp, 1454" and it's well into autumn 1454. Same story with QK, except June hurp instead. Wassapwidat?
EDIT: Screencap below. And yes, Romania - the asterisk is there for a reason, namely that I edited the "Byzantium does NOT exist" condition out of the FormRomanianNation.text file, for obvious reasons.
EDIT2: More data. The truce was extended five more years and I'm not totally sure why. I guess we'll see what happens. The truce with Tamerlane and friends is going to end soon as well.
EDIT3: More data again. Timurid truce expired and the same thing happened; no war, no ability to declare war. I'm not allied to Georgia or Armenia, and I suspect that may be the catch here. (edit4: nope. allied, no war)