I have a forum with considerable size (over 1.5 million posts). It ranked well at Google and Yahoo.
After hearing many good things about VBulletin from people here and elsewhere, I went ahead to purchase and install VBulletin (3.8.2 )with VBSeo to my site, replacing the old phpbb forum. I have taken into considerations previously indexed pages (340k pages from Google) , and made 301 redirect to the appropriate new URL. Eventhough not perfect, I manage to redirect at least 95% of my traffic to the new URL.
Problem is, search engines seems to have problems assigning ranking to the new URLs. The indexed pages drops daily, now 280k pages at Google. The traffic drop even worse. From previously average 12k visitors a day, now I only get 5k a day. Needless to say, my AdSense earnings suffers a massive drop, now with only a little more than one third from previous earnings.
Hope things can turn around, if not I think I'll have to migrate back to phpbb.