Hello,
VM Invoice does not have own table for storing orders. So all orders entered through VM Invoice interface are also visible in VirtueMart as if they were added using a standard checkout process.
I believe the stock levels updating was fixed already.
I am not familiar with the component from Joomla Cache you mention, so I cannot tell. We try to keep ours quite simple, focus on easy way to handle what its supposed to do. That is generating PDF invoices and possibility of manually accepting phone/mail orders.