thermos vs vacuum flask

thermos and vacuum flask both are nouns.

Word	Noun	Adjective	Verb	Adverb
thermos	Yes	No	No	No
vacuum flask	Yes	No	No	No

As nouns, vacuum flask is a hypernym of thermos; that is, vacuum flask is a word with a broader meaning than thermos:

thermos: vacuum flask that preserves temperature of hot or cold drinks
vacuum flask: flask with double walls separated by vacuum; used to maintain substances at high or low temperatures

Other hypernyms of thermos include vacuum bottle.

thermos (noun)	vacuum flask (noun)
vacuum flask that preserves temperature of hot or cold drinks	flask with double walls separated by vacuum; used to maintain substances at high or low temperatures