I think this isn't really necessary, because of:
Thread Tools -> Subscribe to Thread
You can choose multiple ways to stay up to date with the thread you subscribed to, via User CP or Email. A special Notification wouldn't be necessary, click on your User CP and voila, you can easily check if your thread has been updated since your last visit.
So my answer to this suggestion is: No, luxury issue, I don't even know if Vbulletin is capable or if someone has written a script to activate this within the forum board