I've seen a couple minutes ago a post that, in my opinion, needs improvement. I immediately thought of flagging, but when I opened the flag window I noticed the option "very low quality" wasn't there (I didn't want to use that flag anyway, since the right one was "needs improvement"). Has it been removed? If that's the case, why? If not, why can't I see it in the options?
It has not been removed, but if a question is already in the Close Vote review queue, you can't flag it as Very Low Quality anymore. The 'negative' review result for questions in the Low Quality Posts is a vote/flag to close, so it's better to have the question in the right queue (Close Votes) immediately.
Also, if a post is at least seven days old, it's not available either: see Shog9's answer here. It may also not have a positive score.