Thanks Milbo!
I will consider a membership.
But unfortunately, when i saw the bug and no quick solution, i opted for the suggested method of giving a manufacturer to all of my products.
Anyway, i "recreated" a similar category structure, and all problems seems to dissapear except for this:
-unpublished category name appended to the page/tab name
-breadcrumb of the product is under root instead of the parent category
-SEF url has the same problem as the breadcrumb
The first problem can be solved changing /components/com_virtuemart/views/productdetails/view.html.php :
$document->setTitle(strip_tags(html_entity_decode(($category->category_name ? (vmText::_($category->category_name) . ' : ') : '') . $product->product_name,ENT_QUOTES)));
to this:
$document->setTitle(strip_tags(html_entity_decode(($category->category_name ? ($category->published ? (vmText::_($category->category_name) . ' : ') : '') : '') . $product->product_name,ENT_QUOTES)));
And I think the two last are not really problems, its the regular behaviour, as the product in fact dont have any published category... anyway, it would be better to keep the category of the parent for both the URL and the breadcrumb. Just a suggestion.