Since VM 2.0.22 it is possible to add products with 0 price to cart. But it is not possible to delete these orders. In this case I get the error message:
vmError:
vmError: VirtueMartModelOrders::remove 399
Bestellungen konnte nicht gelöscht werden.
Only the name of the customer is deleted.
No problem when there is additionally a non free product in the order.