I have problem already suggested this but not 100% sure.
Regardless, it would be great to add a notification alert when someone quotes your post or mentioned you in the post. This is a plugin that I am sure is available for vBulletin (you are using vBulletin afaik). Its simple and easy modification and would only benefit the forum.
Yes you can subscribe to a thread, that is besides the point since that is also available on many other forums/forum platforms but that does not stop them from adding the quote like notification.