I have been gone for 1 year and 8 months, but have only missed 100 new posts in all that time. The numbers do not lie. 3rd party development of vBulletin is dead. I do not regret leaving vBulletin for WoltLab Burning Board and Invision Power Board, where I can still find 3rd party modifications, styles, add-ons, and plugins either free or made to order.
Apparently your method is flawed, you've missed out on over 600 posts in one years time in one forum alone. And this is just using the sort by one year feature. And the numbers do quite ironically lie in your case.
By default threads are only remembered to be new/old for about 2 weeks. There's no way to really say how many were missed but it was quite surely in the thousands if not tens of thousands.