neck opening vs V neck

neck opening and V neck both are nouns.

Word	Noun	Adjective	Verb	Adverb
neck opening	Yes	No	No	No
V neck	Yes	No	No	No

As nouns, V neck is a hyponym of neck opening; that is, V neck is a word with a more specific, narrower meaning than neck opening:

neck opening: an opening in a garment for the neck of the wearer; a part of the garment near the wearer's neck
V neck: the neck of a garment has the shape of the letter V

Other hyponyms of neck opening include neckline.

neck opening (noun)	V neck (noun)
an opening in a garment for the neck of the wearer; a part of the garment near the wearer's neck	the neck of a garment has the shape of the letter V