Placate Vassal Subject option should not be available while the Vasall has below 50% Liberty Desire.

[tobias.mb]

The reason is the "improve prestige" mission.
Currently you can burn your entire prestige placating vassals which already love you, take the mission and then quickly raise your prestige again.
For medium or large nations it is rather easy to get 50 prestige in a single war.

 

[Nuklearius]

The

 

[tobias.mb]

50 prestige is not so easy to achieve

Yeah... except you can do something like take religious ideas and a +1 prestige advisor. This already puts your prestige resting point to 40 prestige. Finish a "conquer this province" mission on top of it and your resting point is at 60 prestige. So you wouldn't even need to go to war. And there is plenty more ways to gain prestige passively.
Also a misison more useful than free stab? Maybe the ones that give you manpwoer recovery, but they often force you to attack someone you rather wouldn't. So which one are you talking about here?

 

[Nuklearius]

Yeah