Hi,
You can found a solution without HikaSerial. I am not sure that HikaSerial would be the right solution for your case, it would depend.
An order custom field which could be good. If the order custom field is not displayed in the front-end so customer could not edit his value. But the value would be editable in the backend and could be included in the order status notification.
So, the vendor would update the custom field, change the order status and notify the customer.
It is possible to use a download override plugin. At this moment, downloading a file would be just possible when an order is "shipped" (you can change the translation for the order status) and it would read the order custom field in order to redirect the user to the specific link.
I would recommend you HikaSerial if you wanted to create dynamically unique content, like file watermarking with a generated key.
HikaMarket allows you to configure fees for orders and products. So you can set a flat fee for a product, you can set a percentage fee on an order.
A variant, like a classical product could be affected to a vendor, so you can create several variants for several vendors.
You can try HikaMarket in our demo website (login demo / password demo) for front-end and back-end.
So I think that HikaMarket would provide you all required features.
Regards,