root vegetable vs veggie

root vegetable and veggie both are nouns.

Word	Noun	Adjective	Verb	Adverb
root vegetable	Yes	No	No	No
veggie	Yes	No	No	No

As nouns, veggie is a hypernym of root vegetable; that is, veggie is a word with a broader meaning than root vegetable:

root vegetable: any of various fleshy edible underground roots or tubers
veggie: edible seeds or roots or stems or leaves or bulbs or tubers or nonsweet fruits of any of numerous herbaceous plant

Other hypernyms of root vegetable include veg, vegetable.

root vegetable (noun)	veggie (noun)
any of various fleshy edible underground roots or tubers	edible seeds or roots or stems or leaves or bulbs or tubers or nonsweet fruits of any of numerous herbaceous plant