plus the posts get archived after so long/so many posts.The archive section removes the links so if you only doing it for backlinks then you better keep posting.
Even if they are archived the original thread remains. The way vbulletin works is that two pages are made per thread, one in the actual forum and one in the archive. The archive links back to the original thread (provided the admin didn't modify this) so the original is always there. Though you are correct, the archive does remove links within the thread.
No, what that does is tells Mediapartners (Adsense bot) to visit the archive'd page and deliver ads based on that page and not the actual thread itself.
Thanks for the info. This was somthing that I was wondering but I was not aware that diluting the results reduced the quality of the one way link. I was under the impression that the links stopped counting from a particular domain after a certain number.