While this is possible, I would not recommend it at the time because there are long-standing bugs in vBulletin that make certain things not work correctly. If you're running vBulletin 4.1.4 or later, the editor will be completely unusable if different forum components, like blog, forum, or CMS are on different sub-domains. This issue is unfortunately inherited by VaultWiki.
Instead, you may just want to redirect visitors to wiki.domain.com to your new wiki address. You can do this by fiddling with your .htaccess file on your subdomain. Example:
Although it may be more beneficial to set up Simple URLs in VaultWiki: Server Settings to have forum.domain.com/wiki/Article
RewriteRule ^wiki/(.*)$ http://forum.domain.com/showwiki.php?title=$1 [L,QSA,R=301]