vacuum flask vs thermos

vacuum flask and thermos both are nouns.

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

As nouns, thermos is a hyponym of vacuum flask; that is, thermos is a word with a more specific, narrower meaning than vacuum flask:

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

Other hyponyms of vacuum flask include Dewar, Dewar flask, thermos bottle, thermos flask.

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