vBulletin is much more SE friendly out of the box then phpbb. You have to modify phpbb to be search engine friendly (at least in version 2.x).

With mods such as removing session IDs and using mod_rewrite to rename your forum and thread pages with .html extensions, I see no reason why phpBB can't be just as SEO friendly as vBulletin. I don't think that it's realistic to expect that if you switch to vB tonight, miraculously your site will rank #1 tomorrow.

Yeah, the mods exist. The big issue with that is every time you upgrade your software you have to reinstall the mods. I think phpbb 3.0 is SE friendly out of the box but don't quote me on that.

sure VB is seo friendly there is no doubt for that.

That sucks. Looks like a hack will be required to correct that again.

I went from fully modded phpbb to vbb with a mod called vbseo. I noticed a night and day diffence. Imo it was worth it for my business. Because i went from being 100 or so in google to number 1 and i show up for all sorts of odd ball search terms.

Even without the mod you would get results because phpbb is not SE friendly out of the box. But if you have used a mod in it it would do just as well as vBulletin. Kind of not fair to go out of your way to make one SE friendly then say it is so much better then the one that you didn't make search engine friendly.

My best in SEO terms is a PHPBB forum. Was never able to make IPB or VB work from this point of view. I also have some bad SEO-d phpBBBs and myBBs .. in the end I think it's more our fault ...

Are you using 301 redirects to point the old URLs to the new ones? If not, the decline is most likely a result of all your pages in google's index no longer being valid.

> However the problem was that there were about a thousand old URLs so I didn't really find any satisfactory explanation on how to do a 'blanket' redirect on all of them...

If threads have the same ID number with both forum systems, use an .htaccess file to point the old URLs to the new URLs. If threads don't have the same ID number, create a redirectional page at the old URL location which fetches the new ID number of the thread and redirects the user there.

However if I didn't do the redirect correctly would the hits not have started dropping off instantly?

You said it yourself - you have over a thousand URLs in Google's index. Google doesn't re-spider all of these pages every day. Depending upon your PageRank, whether you use Google Sitemaps, and many other factors, expect your homepage to be spidered every few days and other pages (such as forum threads) every few weeks to every few months to even less often, in some cases. When looking at the big picture, you would expect a steady decline over the course of about a month (give or take depending upon many factors). I assume that your homepage has always been accessible. So it would take awhile before Google started respidering old forum threads and finding them broken one after another. It would take even longer before Google came across old forum threads that actually had a high ranking. It might have been that for the first two weeks, the pages Google was respidering weren't getting much traffic but then after two weeks, Google got around to respidering a page that was ranking very well.