5 of 6 added 98 characters in body

# Jelly, 13 12 11 bytes

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

Œlf“¡ẎṢɱ»LḂ


All test cases are at TryItOnline

How?

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)

fØcLḂ


Test cases also at TryItOnline

Same as above, but Øc yields the English vowels, 'AEIOUaeiou'