Some VnVs do have triggers, some are random. The VnVs that heirs get when they mature are a bit of a mystery. I think many of them depend on the size of the kingdom and the VnVs and general competence of the faction leader.

I think most of the VnVs that heirs/generals acquire as they age have triggers, either due to their actions/inactions or side effects of the empire's success. Jxrc put together a nice PDF with trigger observations in this thread (last post). I haven't had a chance to update the VnV wiki page yet with the info though.

Welcome to the .Org, Tirpitz!