There should be a bridge for vB3 in Coppermine 1.3 already.
You just go to the CPG/bridge/vbulletin3.php, open it and edit the settings, then open CPG/includes/init.inc.php, and uncomment the vB3 line.
Then all your users will be able to use the gallery fine.
To edit the theme, open CPG/theme/default, and edit theme.html so it matches vB.
Now, skinning the actual image gallery itself can be pretty difficult, but still.. It's free, and it works.