this was a topic already but i didn't know how to post on it
i think a vowel is a letter with a short sound that isn't accompanied with another letter. the y sound in "yes" is "ya" the y sound in "sky" is iya without the "a" sound a e i o u all make a short and single letter sound if you think the short "e" sound is eh its not the h is silent and isn't part of the sound. this is just my oppinion and i may be completely and totaly wrong.
I think that the best idea is to refrain from using the terms "vowel", "consonant", "semivowel" when you're talking about letters. Reserve those terms for the sounds of speach.
You would then say not that the letter "i" is a vowel but that the letter "i" represents a vowel in a given context. What do I mean by "context"? In the word "pin" the "i" represents a vowel but in the mathematical formula "i^2=-1" it represents a number. The "I"s, "V"s and "X"es on the face of a clock do not represent vowels or consonants.
Of course this gives you a very round-about way of talking about the letters. The way I get around this is to use expressions like "vowel-letter" and "consonant-letter". You could also use the expressions "orthographic vowel" and "orthographic consonant". Then on the other hand, sometimes it's better to use the round-about way after all.
The letter "y" represents a (phonological) consonant in "yes", "year", "yet", etc. The same letter represents a vowel in "sky", "cry", "happy", etc.
Then what is a vowel? It's a sound made (by the human voice) with no (or relatively little) obsruction of the vocal tract. Of course this is a bit of an over-simplification and there might be a distinction to draw between phonetic and phonological vowels but let's not get too carried away.