Like davidjoly, I found vm search autocomplete easy to install and configure. Adds a great feature to the e-shop with the name and thumbnail image of the products showing up as the user type!
Using it on 2k products virtuemart 2.6.6 on Joomla 2.5.
If you are looking for Search suggestions for VM this is your tool! I had some minor issues with it(some caused by template) and the developer quickly fixed them. I also requested a feature whereby clicking the suggested product would take you directly to the product page, the developer added the feature and sent me the new plugin within hours. Greatest support ever! Many thanks. Rich