I have searched the forum and did not found any trick or tip about the subject ; this is why I wrote and post this subject here.
There are 2 ways to do it and they are explained here :
http://www.webtimarket.com/en/help-for-virtuemart/133-not-displaying-product-filter-in-virtuemart-2-category-page.html (http://www.webtimarket.com/en/help-for-virtuemart/133-not-displaying-product-filter-in-virtuemart-2-category-page.html)
hope that can help some of you!