I'd have to agree with Bingley on the w part. I've never even thought of considering w a vowel. Whenever a child recites the vowels he doesn't say a-e-i-o-u and sometimes w. Just because it's silent in words such as hollow doesn't make it a vowel, it's not producing any sound, it's just there because that's how they liked to spell it in Old English, and it's just there for looks now.

If we're going to consider w a vowel, then we'd have to make h a vowel as well, because most people don't pronounce the h in honor. These silent letters are just there to balance the word and make it look better. And remember, we have the silent e also.

For the most part, I'm hesitant to consider y a vowel as well, but I'd say that such usage originates in other languages, most likely Welsh.