you cannot call strlen() on something like

char vowels[] = {'a','e','i','o','u'};

This is a char array, not a null-terminated string. strlen() works by iterating through the string until it finds a nul byte ('\0'). vowels does not contain one, so it will keep scanning the memory until a nul byte happens to be there by chance. this is undefined behavior and should be avoided at all costs. In the sample tests you are just being lucky.

Also, performance note: you should almost never call strlen() in a loop on the same string: this makes your program slower for no reason; because unless the string's length is modified during the loop, strlen() will keep doing the exact same operation again and again and return the same result every time