Vowels are letters that can “say their name” when used in a word. Actually, we do a disservice by teaching the alphabet by name to a child first, as in the ABC song. I taught my child the sounds first, in the order of the alphabet, and he learned the song later in school. I think it helped a great deal, as he learned to read much earlier than his counterparts. Just tell your child that when letters are used in words, they each have special sounds. The vowels are special because they can also say their names. Then give examples.
Go through the alphabet by writing them out in lower case and make the vowels a different color, usually red vs blue, that way they stick out. Say each one as you go, pointing out how the vowels can be different.