Try to compress images and show less per page
Image which is 125 kb could be compressed with Photoshop.
- Save images for website.
- Make a jpg at 60 quality (where difference is hardly noticeable).
- The image size is reduced to 42,2 kb.
Use the 'Firebug' plugin in Firefox
Open your page in browser FF, turn on Firebug click on the 'Net' tab and 'All', then refresh the page. It will show you the loading times of all your page elements.
CDN plugins could give really good results in speed tests.
I tested with pingdom.
However, do more than one tests because caching on CDN servers may take some time.
Try without the slideshow or less images in slideshow.
Slideshow could load a lot of files.
Avoid Google webfonts and Google jQuery.
Using the Google jQuery often produces problems.
The version they use is different from the default Virtuemart jQuery version.
Try without cufon with externally hosted (slow) fonts.
It could give 10 seconds speeding without all this stuff.
Compress (optimize) CSS files
Turn Gzip ON.
Updating Virtuemart could give some percents of speed.
It could be useful to edit old template override to NOT call the old modal code for product detail page.
Virtuemart has switched to fancybox.