Which vowels should singers practise?
Most singing teachers start from five pure vowels, written in the International Phonetic Alphabet as [i] as in "see", [e] like the start of "say", [a] as in "father", [o] like the start of "go" and [u] as in "boot". Practising scales and held notes on each one builds an even tone across vowels, which keeps words clear and the voice consistent in a real song. Vowels change shape as the pitch rises, so working each one through your range is part of extending that range smoothly.