I think I figured it out.
I think what threw me is that the installation instructions had two different directories:
* root/forums/includes/xml - cpnav_vbagallery.xml
* root/forum/modcp - vba_gallery.php
"forum" vs. "forums"...I wasn't sure if I was supposed to have/create another directory.
It might be less confusing, in the installation instructions, to say something like:
Rename the vBgallery 'forums' directory to the name of your vBulletin forum.
Then transfer the renamed vBgallery directory to root directory.