A point system seems gimmicky and boring. I don't want to feel like I'm playing a board game, I want it to feel like real diplomacy. For example, what I do is raise taxes while the vassals are doing well financially and lower them when they are doing poorly financially. I sometimes go as far as give them gifts if they are doing very poor (vassals will still be "guarded" with me, though, no matter how generous I am with them). It might be better to have something more interactive like that, wherein raising taxes while they are financially well-off doesn't affect how content they are as much as if you raise it while they are worse off and cutting it while they are doing poorly financially is more rewarding than if you did it while they were doing well financially. Perhaps that's already how it works.