Welcome to the Org, Inbetween.

Indeed it is your despicable reputation, and nothing else it seems, which destroys the ability to ally with the computer controlled factions.

Remember that declaring war against members of the same faith, sacking, pillaging, or exterminating cities, not releasing prisoners, and a whole host of other reasons will destroy your reputation. No one will ally with you, even if they are guilty of the same thing.

It is indeed possible to get vassals, but you have to be on good terms with them and have a sparkling reputation, and they need to be nearly defeated and at war and you probably need to offer them valuable territory to get the vassalhood. Plus it's hard to be on good terms with those you're at war with.

I will defer to others who have more experience getting vassals. All I can tell you is that I was a chivarlous leader at one point, and now... I don't have any use for vassal states.