Comparison of char in C

This syntax is completely wrong, you have to compare the variable against the character individually.

if (atual->letra == 'a' || atual->letra == 'e' || atual->letra == 'i' || atual->letra == 'o' || atual->letra <= 'u')

I was comparing the first boolean expression against characters. A non-null character is a value other than 0, and in Boolean logic 0 is false and all other values are true, so already in the second expression after || will always give true, which is not the desired one.

How about using a function to check if the letter is a vowel:

int eh_vogal( char c )
{
    int i = 0;
    static const char v[] = "aeiouAEIOU";

    while( v[i] )
        if( v[i++] == c )
            return 1;

    return 0;
}

With this, your code would look like this:

int BuscaNv(LISTA* resp){
    NO* atual = resp->inicio;

    while(atual){
        if(eh_vogal(atual->letra)){
            printf("É vogal\n");
            atual = atual->prox;
        }
        else{
            printf("Não é vogal\n");
            atual = atual->prox;
        }
    }
    return 0;
}

If you do this if yours works like this?

    if(atual->letra == 'a' ||
       atual->letra == 'e' ||
       atual->letra == 'i' ||
       atual->letra == 'o' ||
       atual->letra == 'u'){
    ...
    ...
    ...
    }