If you create a category and set its 'Parent Category' to itself, it vanishes from the Product Categories list.
If you browse to it at the front end you get a warning "Stopped getCategoryRecurse after 10 rekursions" which makes sense, but you can't go into the category in the back end to fix it.
Go into phpmyadmin and change it in xxxx_virtuemart_category_categories.
Jörgen @ Kreativ Fotografi
It's not a problem I currently have; I was fortunate enough to have another browser window open with it still in, but it just seems like 'undocumented behaviour'
Thx, fixed by adding
if($data['category_parent_id'] == $data['virtuemart_category_id']){
$data['category_parent_id'] = 0;
}