Game concept
Offer Vassalization
The game also links it as Offer Vassalization Interaction.
From the game's encyclopedia
An interaction offering Vassalage to an Independent Ruler of lower Rank, peacefully adding them to the realm.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Offer Vassalization is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An interaction offering Vassalage to an Independent Ruler of lower Rank, peacefully adding them to the realm." It is part of Interaction. The game also links it as Offer Vassalization Interaction. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.