Actually which forum we choose to use on our site has nothing to do with our software it is merely personal choice on the owners part. Upgrading the forum would mean having to redesign the site again which he is not prepared to do at this point.
Remember vbulletin 4 architecture is totally different than vb3 so there are many things in use here that may not be compatible. vbulletin 3 is still a valid product. Our software supports 20 different products not just vbulletin 4.
Possibly in the future he may decide to change what product he uses for his support forums but the support forum is merely one aspect on the site which has many parts of the whole which fits the design of the site.