When is y considered a vowel?

I'm not sure it's considered a vowel ONLY when there is no other vowel in the word. I'd say it's considered a vowel when it is used as and pronounced like a vowel. This mostly happens in foreign-origin words (often from Greek, where y is a vowel). Examples would be sybaritic (pronounced as if it were spelled "sibaritic") or rhyme (pronounced as if it were spelled "rime").