6 added 9 characters in body
source | link

Jelly, 13 12 11 bytes

-1 byte thanks to @Luis Mendo (use 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 EnglishLatin alphabet's vowels, 'AEIOUaeiou'

Jelly, 13 12 11 bytes

-1 byte thanks to @Luis Mendo (use 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'

Jelly, 13 12 11 bytes

-1 byte thanks to @Luis Mendo (use 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 Latin alphabet's vowels, 'AEIOUaeiou'

5 added 98 characters in body
source | link

Jelly, 13 12 1211 bytes

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

Œlf“aeiou”LḂŒlf“¡ẎṢɱ»LḂ

All test cases are at TryItOnlineTryItOnline

How?

Œlf“aeiou”LḂŒlf“¡ẎṢɱ»LḂ - Main link takes an argument - s
Œl           - lowercase(s)
   “aeiou”“¡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'

Jelly, 13 12 bytes

-1 byte thanks to @Luis Mendo (use to replace %2)

Œlf“aeiou”LḂ

All test cases are at TryItOnline

How?

Œlf“aeiou”LḂ - Main link takes an argument - s
Œl           - lowercase(s)
   “aeiou”   - string of lowercase vowels
  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'

Jelly, 13 12 11 bytes

-1 byte thanks to @Luis Mendo (use 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'

4 added 69 characters in body
source | link

Jelly, 1313 12 bytes

-1 byte thanks to @Luis Mendo (use to replace %2)

Œlf“aeiou”L%2Œlf“aeiou”LḂ

All test cases are at TryItOnlineTryItOnline

How?

Œlf“aeiou”L%2Œlf“aeiou”LḂ - Main link takes an argument - s
Œl            - lowercase(s)
   “aeiou”    - string of lowercase vowels
  f           - filter - keep only the vowels
          L   - length - the number of vowels
           %2 - Bit (modulo 2)

Non-competing, 65 bytes (since I just added the function Øc)

fØcL%2fØcLḂ

Test cases also at TryItOnlineTryItOnline

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

Jelly, 13 bytes

Œlf“aeiou”L%2

All test cases are at TryItOnline

How?

Œlf“aeiou”L%2 - Main link takes an argument - s
Œl            - lowercase(s)
   “aeiou”    - string of lowercase vowels
  f           - filter - keep only the vowels
          L   - length - the number of vowels
           %2 - modulo 2

Non-competing, 6 bytes (since I just added the function Øc)

fØcL%2

Test cases also at TryItOnline

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

Jelly, 13 12 bytes

-1 byte thanks to @Luis Mendo (use to replace %2)

Œlf“aeiou”LḂ

All test cases are at TryItOnline

How?

Œlf“aeiou”LḂ - Main link takes an argument - s
Œl           - lowercase(s)
   “aeiou”   - string of lowercase vowels
  f          - filter - keep only the vowels
          L  - length - the number of vowels
            - 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'

3 added 336 characters in body
source | link
2 added 406 characters in body
source | link
1
source | link