V neck vs neck opening

V neck and neck opening both are nouns.

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

As nouns, neck opening is a hypernym of V neck; that is, neck opening is a word with a broader meaning than V neck:

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

Other hypernyms of V neck include neck.

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