The "long" versions of the five traditional vowels aren't even pure vowels, but diphthongs: /ɛi, i:, əi, ɔu, ju/. The letters [i]y and w are sometimes called semivowels, because than can be vowels or consonants. In English, w is a vowel only in a few Welsh loanwords, e.g., cwm. If we're talking how many vowels (vocalic phonemes) does English have phonologically, the answer is usually 23 for RP and 19 for General American, (though these numbers include diphthongs).

Ceci n'est pas un seing.