Determine the vocab from the qualifier.

def determine_vocab(self, qualifier):
        """Determine the vocab from the qualifier."""
        vocab_value = VOCAB_INDEX.get(self.tag, None)
        if isinstance(vocab_value, dict):
            if qualifier is None:
                qualifier = 'None'
            # Find the value based on the qualifier.
            return vocab_value.get(qualifier, None)
        elif vocab_value is not None:
            return vocab_value
        else:
            return None