Although a hack is possible, it would somewhat duplicate a default vBulletin feature. Before I'd hack my system, I'd give the following a try:
For each user who wants to be notified, add that email address to one of both of the following fields in the forum's properties. The fields are located under "Moderation properties."
Emails addresses to notify when there is a new post
Emails addresses to notify when there is a new thread
Of course, you may have 50 forums to apply this to for all I know.
This isn't a great alternative. For one, you'll receive emails where the URL will produce an "invalid threadid" message.
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