Hi,
The order_vendor_id is only used for sub orders where the vendor is always unique.
For the main orders, as they can potentially be configured with products from different vendors, that value stays at 0.
Even in "one vendor" mode, you are still able to add products from different vendors in the backend, so it doesn't change the way it works and that's why what you want doesn't work.
The only solution I see is to develop a small plugin in the group "hikashop" and implement the onBeforeOrderNumberGenerate(&$order, &$result) trigger
In $order you'll have the data of the order, and in $result you can set the order number you want to be used.
That way, you can use the vendor_id set in the products (I suppose in $order->cart->products).