How can y sometimes be a vowel?

The previous answers are correct - it's part of the problem of having many languages sharing the same written alphabet - it has to be adapted. The letter 'w' is used as a vowel in some languages, eg Welsh, but in others is a 'double vee'. Strictly speaking, w would be a semi-vowel in English, as when used in a word, the lips almost come together, but not quite.