I noticed a similiar problem and I wonder if there's a fix.
I'm currently using VaultWiki 3.0.20 with vBulletin 4.2.1.
Now I wanted to add a Widget that displays all new Wiki-Articles of a certain Wiki-Forum using the recent post widget of vbCMS (Template "vbcms_widget_recentposts_page")
However when I try to configure the widget, the Wiki-Forum is not displayed in the list and thus not selectable. Similiar to nasko's problem mentioned above.
And yes, I entered a link to the Wiki's Main page in the forum so that's the reason why it's not displayed.
When I delete the link, I can select the board in the widget's configuration menu properly. However, I need a link to the main Wiki page.
Now I'm wondering if there's a fix or a workaround? I tried adding the fix mentioned above (vault/plugins.php) and it works perfectly for the search, but obviously not for the widget. Could you please help me out, Pegasus? That would be great!