The workers have formed a union, In this sentence 'u' is a vowel than how the sol came as 'a'

The article ‘a’ should always be used before words that start with consonant sounds. 
So, you need to remember that if a word begins with a vowel, but has an initial consonant sound, (‘university’ as an example), use the article ‘a’ before it. Examples of consonant sounds of words which begin with a vowel are: a university, a unique, a euphoria etc.