Why is the option very low quality sometimes hidden in the flag menu?
About a moment ago, I saw a post of very low quality on Stack Overflow, and I wanted to flag it as very low quality but this option was not in the flag menu.
The low quality flag option will come up for zero or negatively scored posts - that's the first indication that a post is indeed low quality (or suffers from other issues).
I can only assume that the post had no downvotes at the time (or enough upvotes that the score was positive), and as such, the option didn't come up.
Why flag before voting? Voting is the first and most important thing to do.
As of June 2016, another reason for the missing VLQ flag option is that the post (question or answer) is older than seven days. If a post sticks around for 7 days or more, chances are very high that it's not very low quality:
but the truth is that very few VLQ flags ever get raised on posts over 7 days old... Heck, 87% of all VLQ flags are raised on questions less than one day old. The vast, vast majority of the worst cruft gets cleaned up quickly...