f4c6237e_Diplomacy___EU3_Wiki__Diplomatic_Action
[Prerequisites] Target must have been a vassal for 10 years, you did not annex another vassal for 10 years, relations ≥ 190 [Effects] Vassal is incorporated into your nation, +1 move to Decentralisation (), -1 Stability () [Diplomatic Action] 
Demand Annexation (Commonly called diplo-annexing)
Diplomacy - EU3 Wiki
Diplomatic Action