If an answer has no references, should I use the "Very Low Quality" flag, or "in need of moderator attention" flag?
No references is not mutually exclusive with VLQ. It can have no references and be quite good (albeit this would not be an acceptable answer here!), it can have references and be quite bad.
I would say use your intuition and use whichever is most appropriate on a case-by-case basis.
In addition to JamieC's answer:
Don't hesitate to also add your own comment. Here is the template I use:
[Welcome to Skeptics!](https://skeptics.meta.stackexchange.com/questions/1505/welcome-to-new-users) Please [provide some references](https://skeptics.meta.stackexchange.com/q/5) to support your claims.
There are some more suggested templates and actions here: Guidelines for Inadequately Referenced Answers