6 of 6 added 9 characters in body

Jelly, 13 12 11 bytes

-1 byte thanks to @Luis Mendo (use to replace %2)
-1 byte thanks to @Dennis (use a string compression)


All test cases are at TryItOnline


Œlf“¡ẎṢɱ»LḂ - Main link takes an argument - s
Œl          - lowercase(s)
   “¡ẎṢɱ»   - string of lowercase vowels (compression using the words a and eoui)
  f         - filter - keep only the vowels
         L  - length - the number of vowels
          Ḃ - Bit (modulo 2)

Non-competing, 5 bytes (since I just added the function Øc)


Test cases also at TryItOnline

Same as above, but Øc yields the Latin alphabet's vowels, 'AEIOUaeiou'