thralldom vs vassalage

thralldom and vassalage both are nouns.

Word	Noun	Adjective	Verb	Adverb
thralldom	Yes	No	No	No
vassalage	Yes	No	No	No

As nouns, vassalage is a hyponym of thralldom; that is, vassalage is a word with a more specific, narrower meaning than thralldom:

Other hyponyms of thralldom include bonded labor, servitude, serfdom, serfhood.

thralldom (noun)	vassalage (noun)
the state of being under the control of another person	the state of a serf