Sorry me again : apparently, the breadcrumb works ok if we are using it from standard browse page. However, all my products are also introduced via VM Latest Products module and breadcrumbs are not displayed on product page when coming from that module.
I finally found the problem on this issue, the breadcrumbs are dispayed only if the source page is authorising them : on homepage, there was no breadcrumbs displayed, clicking on a product from there makes it imposible to display breadcrumbs on the related product page. The solution was to display breadcrumbs on the homepage.