thermos flask vs vacuum flask

thermos flask and vacuum flask both are nouns.

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

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

thermos flask: 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 flask include vacuum bottle.

thermos flask (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