One of the foremost things you want of your own vBulletin forum is that the users can engage in it as much as they would. Though email subscription is a little too bothering than people need, control panel subscription that lists all the threads your users have started or participated in will definitely increase the vitality of your community because they can easily find out if their threads / posts have been responded to and then feel compelled to post back.
The default subscription mode when a new user registers with vBulletin is no subscription at all. People would have no way to find out how their posts have been received and responded to thus would become engaged in the conversations. You should turn the control panel subscription on by default so all new users will subscribe to their own threads or threads that they have posted in automatically.
Log in your vBulletin control panel:
Admin CP –> Settings –> Options –> User Registration Options –> Default Registration Options –> Automatic Thread Subscription Mode
Select the dropdown list to Subscribe with no notification. And click Save. However, for users who have already registered, they’d have to manually change their account settings to change their subscription options:
User CP –> My Settings –> My Account –> General Settings –> Messaging & Notification –> Default Thread Subscription Mode
Change it to Through my control panel only.