oddly I had exactly this problem yesterday with a site upgrade...
and this is the weird part...
In Joomla admin config it clearly showed mysqli in the dropdown but VM still said mysql on install.
The installer takes this directly from the configuration.php
$config = JFactory::getConfig();
$type = $config->get( 'dbtype' );
if ($type != 'mysqli' and $type!= 'Jdiction_mysqli') {
JFactory::getApplication()->enqueueMessage('To ensure seemless working with Virtuemart please use MySQLi as database type in Joomla configuration', 'warning');
return false;
}
when I checked the configuration.php by ftp it did say public $dbtype = 'mysql';
I have no idea how this can be possible but manually changing this to public $dbtype = 'mysqli'; and the upgrade worked fine