Hi, please take a look at my post below. After speaking to Webkul who make the extension 'Marketplace for virtuemart' they told me that it is possible with a customisation but that they don't have time to do it and that any other agency or developer can do it.
So if there is anyone out there who thinks they can do this before my site is launched on Wednesday please let me know. Thanks.
https://forum.virtuemart.net/index.php?topic=145443.0