I don't think there is an over-thought here.
I just have users that are looking for a "feed" that alerts them of
any activity that takes place (be it new threads, new posts, etc) within a sub-forum --- Without ever having to manually subscribe. The user would only subscribe
once to the forum/sub-forum they choose and get all the updates they need.
The daily digest which is built in works. And this plugin for instant thread-only notifications works. But vBulletin has no way of notifying users of new posts in existing threads... threads that the users are not aware of. Users have to subscribe to each individual thread manually if they want instant updates.
I am guessing that this isn't a feature because of how busy the server would be with every post alerting users who subscribed instantly. But this particular kind of "feed" notification would be used in a forum with low activity, and low post count.