Hello,
I'm sorry for the delay.
Our VM Invoice component is used to generate customized invoices in PDF format for orders from VirtueMart, so it can be used instead of VirtueMart's own invoicing system. Our component provides more options of customization of the invoice than VirtueMart itself.
The second main functionality of our component is that it allows you to enter VirtueMart orders from the backend.
If I understand correctly your use-case, you have multiple vendors who have access to your Joomla's backend and you would like them to be able to enter orders to your system for 3rd party end clients, who are not registered on your website. Is that correct?
In that case, they could use VM Invoice to enter the orders, but there's no concept of "vendors", so e-mails would be sent only to end clients and to one specified e-mail of the main seller. The other problem would be, that each "vendor" would see all the orders of other vendors and could modify them, which is probably not what you want.