The way the vBulletin Developers designed it, was that all the options (which would include Thread Subscriptions) are on the proper (advanced) reply form. Thus, only the essential buttons featured on the Quick Reply.
Are you asking if you can permanently disable Thread Subscription (you can do this in the User CP)? Or do you want to subscribe to some select threads (you'd have to 'Go Advanced')?
Alcar...
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