It depends on how you are defining the media calls in the VIPP file. FFCore can convert VIPP to PDF/vt. If the media calls in the VIPP file are retained within the PDF/vt metadata in a manner that matches the spec in the Press SDK then you would not need the VIPP license in FFPS.
However, there are multiple ways to specify media calls in VIPP and this would only cover the case in which VIPP is built to transfer the media constructs to PDF/vt metadata. This is a question that someone from XMPie would have to answer.
If I have XMPie uDirect Classic, and use it to export a dynamic print VIPP with varible stocks, and send it to a 2100 Xerox printer via Free Flow Core that does have the variable module, do I still need VIPP license on the Free Flow Print server driving the 2100?