I can't see this happening as phpnuke and vBulletin use their own, unique template systems. AFAIK, there is no way to integrate the two into one template system and the same can be said for most CMS' out there when used with vBulletin.

There are a few CMS' that will use the same template system as vBulletin such as Subdreamer, vBAdvanced, etc.

The only thing I can suggest is to find either a phpnuke theme or a vBulletin theme you like and alter them to match each other as much as possible.