"Ambitious vassals at the border" - does the trait actually make them more aggressive? In general you do
not want Ambitious vassals, you want Content vassals. Large opinion penalty for the former, large bonus for the latter. Also, aggressive vassals are very much a matter of taste. My biggest annoyance with CK2 is that my border vassals are always going off and conquering stuff. I want very tight control over what land (if any) I'm conquering, and want to keep to de jure borders. So I try to make my border vassals as weak as possible, and try to block off my strong vassals from having an external border (example in Persia - I keep Hormuz as a demesne province so that the Satrap of Kerman doesn't have a land border with Oman, keeping him from rampaging across the strait (the Silk Road TP is also nice of course
)). In hindsight another nice feature of Sicily/Apulia is that your vassals actually behave themselves - the only land borders are the HRE, the Byzantines, and the Pope. Good luck soloing one of those, AI vassal duke!