Hi,
First of all, you are mentioning;
The site is extremely slow, but your homepage is slow as well.
www.vinidelmondo.nl/nl/
And our VM Template absolutely nothing to do with your homepage, our VM Template start being active when you click any VM product or visit the VM category page. So; if the website is already slow in your homepage, try to eliminate the JS file or plugins you are using in your website layout. This might help to increase the loading performance.
It is clearly obvious that your website is having jQuery conflict issue, this is why lots of things are not working.
First of all, keep this in mind, you can't take all the freeware plugins around and install them all to your website. This is no good. You should carefully pick them and make sure they won't give you headache by loading multiple library to your website or create a conflict.
Disable the page compression tools in your website to clear the source codes, so you can easily see what JS file gets loaded, you know how many jQuery library your website is loading?. Because i just looked at your website, your website doesn't load any jQuery at all. This is insane, your website is full of jQuery plugins, and the jQuery library is missing. Of course none of them would work. Virtuemart officially requires jQuery library to run the "add to cart" function, AJAX loading, and everything.
Open the VM Configuration and make sure jQuery option is enabled, and loading this jQuery from Google Library is better (so enable the external jQuery loading as well). You should disable jquery loading through VM Configuration, ONLY YOU ARE SURE YOUR WEBSITE IS ALREADY LOADING ONE (jQuery1.6) to avoid multiple loading.
Regards.