It is always better to load only 1 jQuery library into a single page. Virtuemart already loads 1 jQuery which is the version of jQuery1.6.4 from external Google server, if you are planing to install some any 3rd party extension, make sure it doesn't load any more jQuery (or manually edit the PHP file to prevent jQuery library loading), because your website is already loading one. (if that particular extension has to load 1 more, make sure that extension has noConflict command and also has some protection to avoid conflict problems; such as; changing the jQuery variable etc..). Usually loading different versions of jQuery in a single page ends up with a conflict problem.