The forum was recently upgraded from vBulletin 3 to vBulletin 4, the latest forum release from vBulletin, although the upgrade went smoothly a drop in the google rankings occured and after much research I discovered why.

I had an expert look at my site and the claim was that although there was no drop in traffic the actual 'power' of the site was reduced mainly due to the size of VB4 compared to 3. The structure of URL's, META etc and the function of VBSEO all stays the same and doesn't effect ranking, however the size of the pages are a major factor. Firstly the calls of js files are much more on 4 than 3, google recomends no more than 2 or 3, but with VB4's base js files, any analytics or adsense and one or 2 plugins and already you are going to slow down.

Also if you install firebug and googles speed test you can see most of the VB4 images can be optimized using firebug, just these images managed to get my site speed from 64/100 to 81/100 so here's a great place to start.

The other major concerns at the moment are....

Combine external JavaScript
Leverage browser caching
Serve static content from a cookieless domain
Remove unused CSS
Use efficient CSS selectors
Combine external CSS
Enable compression
Minify HTML
Minify JavaScript
Minimize redirects

Combine external JavaScript being the biggest issue, although there's plenty of tweeks that can be made to speed up VB4 I don't really like to touch the base code too much as I'm not all that experienced with code but this might be something VB look at when 4.1 is released.

But the expert advise was yes a VB3 site will rank slightly higher than a VB4 although this won't really effect the traffic just the weight of the site as far as googles rankings are concerned.