It is a vowel letter, yes, but not a spoken vowel.
"An" is only used when the following word begins with a spoken vowel.
Give me an A!
Give me a B!
Give me a C!
Give me an H! H = [Age] -> spoken
Give me a U! U = [You] -> spoken
the article 'a' is used for words starting with a consonant sound ( not always a consonant letter) while the article 'an' is used for words starting with a vowel sound (not always a vowel letter). the English language has 5 vowels and 21 consonants from which 20 vowel sounds and 24 consonant sounds can be produced.
the letters 'u' and 'h' usually confuse many learners. 'u' has a consonant sound in universal, union and urinate but has a vowel sound in uncle, umpire, until.
As for the letter 'h', it does not have a vowel form but is silent in words like honour, honesty and hour. this way, the vowel sounds start the words making it possess the 'an' article.
U is pronounced /ju/. the consonant sound there attracts the 'a' article.