Turn ['a', 'i', 'u', 'e', 'o'].contains(c) into:

1 - inline:
char arr[] = { 'a', 'i', 'u', 'e', 'o' };
for(int i = 0; i < 5; ++i) {
    if(arr[i] == c)
        return true;
}
return false;

2 - constant loop unrolling:
if('a' == c)
    return true;
if('i' == c)
    return true;
if('u' == c)
    return true;
if('e' == c)
    return true;
if('o' == c)
    return true;
return false;

3 - multiple comparisons to same variable turned into a switch:
switch(c) {
    case 'a':
    case 'i':
    case 'u':
    case 'e':
    case 'o':
        return true;
}
return false;