I can tell you when I install vbGallery its pretty simple
1. UPLOAD ALL THE FILES ( forum files into the forum and gallery files into your new vbgallery directory )
2. CHMOD THE FILES AND USER DIRECTORIES
3. RUN THE INSTALL PROGRAM.
That is about all I have ever had to do.
So tell me does that file exist? If the xml exists you should not have any issues unless the file is corrupt which is why I asked both questions above.