I can't see the difference bewteen very low quality and not an answer. The flag descriptions are:
it is not an answer
This was posted as an answer, but it does not attempt to answer the question. It should possibly be an edit, a comment, another question, or deleted altogether.
it is very low quality
This answer has severe formatting or content problems. This answer is unlikely to be salvageable through editing, and might need to be removed.
I guess that an useful comment should be flagged as not an answer. But when the answer is just nonsense, is it very low quality or not an answer?
And how do questions get to the low quality queue, since there is no low quality question flag?