From the game's encyclopedia
Your Vassal Taxes consist of the Taxes that you receive from you Vassals. The amount depends on the each Vassal's Obligations.
From your Vassals' point of view, these same taxes are called Liege Taxes to distinguish them from any Vassal Taxes they collect from their own Vassals.
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.
Vassal Taxes is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Your Vassal Taxes consist of the Taxes that you receive from you Vassals." It is part of Taxes. 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.