vowel vs letter of the alphabet

vowel and letter of the alphabet both are nouns.

Word	Noun	Adjective	Verb	Adverb
vowel	Yes	No	No	No
letter of the alphabet	Yes	No	No	No

As nouns, letter of the alphabet is a hypernym of vowel; that is, letter of the alphabet is a word with a broader meaning than vowel:

vowel: a letter of the alphabet standing for a spoken vowel
letter of the alphabet: the conventional characters of the alphabet used to represent speech

Other hypernyms of vowel include alphabetic character, letter.

vowel (noun)	letter of the alphabet (noun)
a speech sound made with the vocal tract open	the conventional characters of the alphabet used to represent speech
a letter of the alphabet standing for a spoken vowel