This is more of a "community norm" kind of question rather than a technical one. It's been a while since I've been involved with an online community, but I'm very familiar with the vBulletin format. I was really surprised to see that threads are not automatically subscribed to when a response has been posted. I searched this topic and found a lot of people have asked in the past "why am I not subscribed to a thread I posted in ...?", so it seems a lot of people have this expectation.
I've used three other vBulletin based boards in the last 11 years, and since those automatically subscribed someone on posting, I'm assuming this is a feature the board admins can enable or disable. In other words, admins control what the default is
when someone registers. I have set my options here to be subscribed in the way that best suits me, but I also know that most people don't know what settings are "options" and don't know they can change this setting - especially new people. Even some seasoned vBulletin veterans refer to a thread page number, for example, not even realizing that not everyone views the default 10 posts per page.
I'd like to suggest that the default be that threads are automatically subscribed to with no notification. That way people can easily find their way back to them by clicking "user CP" but won't be bombarded with unwanted e-mails. It just seems like this would be a way to insure people go back to conversations to which they've posted, and enhance the community experience, especially for new people.
'Course being one of those new people myself, I may be raising a topic that's been debated by admins and moderators and perhaps you have already decided the subscriptions are handled the appropriate way now. In that case ... Never mind! (another Gilda Radner reference for Suzanne.