Just an FYI
The import xml file 'product-vbbuxplaza.xml' has SQL syntax errors in it when used with MySQL 5.1.7 beta. It may be a 5.x thing in general, I don't know.
In any case, the error is in the 'CREATE TABLE IF NOT EXISTS' lines (about line 288 in the file, for example).
The 'type=MyISAM' needs to be replaced with 'ENGINE = MYISAM'
I found this by disabling all the $db->hide_errors() in the xml and saw what the offending error was.
Otherwise, the script runs happily as if everything worked, even though it did not create a single table with prefix vbp_
Another small nit I have with the install is that the page that shows the status of the install/upgrade has a redirect at the end, so it disappears from the browser too fast to really read anything on it. Should be a button to continue.
Hope this helps someone out there
Nice work, though!