An interesting question. I do not think vBulletin has such an option in its search feature. You may be able to get around that, at least for some posts, by doing an advanced search where you put your username under "User Name:" and set it to display "Threads" and not "Posts". Set "Find Posts From" to be a year ago and older. As long as you've not replied to more than 500 threads you will see all the threads you have replied to. Granted you'll have to go through those threads to find your posts but that may work for you.