I think the diplomatics in this game are not so good at all. If you have a vassal, you can't interact with him. Also alliances feels useless, since there is not such kind like defense pacts or shared win conditions. So it ends up you eliminate your vassal or Ally, which feels a bit strange. 

I really would like to see more diplomatic options.

I don’t think we need shared win conditions or defence pack, but I believe that diplomatic screen needs the same treaties tree as IP’s diplomatic screen. So we could develop our alliance in some one way like science, influence, trade or warmonger. Also would be cool if vassal had the same screen, but with some debuffs, so liege had to choose between maximal profit from vassal and vassal’s behaviour on lieges actions towards him