I was looking around the forums and found some posts about people having a hard time getting capitulation from AI that already have vassals themselves. Capturing Vienne+ might be necessary.
Edit: Also note that without capturing Vienne, Bibracte can't use its corn or gold mine. Then we need to at least move some seige units with escorts (fearing HA attack) towards Vienne with all deliberate speed once the SOD is dealt with.
We can prepare the way for capturing Durnovaria by bombarding with our frigate and then sending in 3 Cuirs when such a force is available. I suggest our frigate starts moving around Durno's borders this turn. If we declare war next turn, frigate heads toward Tolosa, but takes one shot at Durno on the way. Then for the next few turns, it bombards Tolosa until we're ready to attack there. Then back to Durno to finish the bombing. After those battles, the frigate can get into position to bomb Indian cities, along with new frigates being produced elsewhere.