vowel vs alphabetic character

vowel and alphabetic character both are nouns.

Word	Noun	Adjective	Verb	Adverb
vowel	Yes	No	No	No
alphabetic character	Yes	No	No	No

As nouns, alphabetic character is a hypernym of vowel; that is, alphabetic character is a word with a broader meaning than vowel:

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

Other hypernyms of vowel include letter, letter of the alphabet.

vowel (noun)	alphabetic character (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