Is it really this hard to get a vassal

[lfhzl]

I played as France,egypt and turkey(all double vh). Every time I had more than 25 regions,those factions which fought me early were very weak,so I really liked to see some of them to be my vassals for some fun multi-factions battles(especially to make some of their regions to be the frontline against mighty mongol army ).But my proposel never worked,even always ended up very demanding.I felt annoyed because these factions were obviously far more weak than mine,not to metion later in these games I was superme of the supermes.always very demanding(I even offered one or two citys and money they could never raise)Why is it so hard to get a vassal.I appreciate any help I can get.

[lfhzl]

sorry for my errors,my computer had some problems when I was posting this thread.so it ended up the wrong area and seveal same threds,please remove it.