Google's Ngram Viewer suggests references to "vassals" have been in steady decline for almost two hundreds years, yet Google Trends suggests a doubling or even tripling of interest over the past twenty. The culprit can be found among the "related topics" list at the bottom, which includes Crusader Kings 2 and Europa Universalis 4.
The relentless PR campaign in favour of feudal obligation continues in the next Stellaris expansion, Overlord, which will add new "vassalization mechanics" when it launches on May 12th.