There already was Sentence case, lower case, UPPER CASE, Capitalized Case, AlTeRnAtInG CaSe and Title Case.

And nOw YOU cAn dO vOwEr cAsE tOO!

function toVowelCase(text) {
  return text.toLowerCase().replace(/[aeiouy]+/g, match => match.toUpperCase());
}

Updated: a correct implementation of toVowelCase is locale-dependend. In English, the letter y is sometimes a vowel, and at other times a consonant. The above works for the Dutch language and some other languages. Your mileage may vary.