The USB-SPI cable is a standard FTDI cable. Examine pages 10 and 11 of this document for details. I'm not sure why Granite Devices calls it an SPI connector when it seems to operate as a TTL level RS-232 connection.
I better qualify my response: I have VSD-E drivers, and the above cable is appropriate for them. I do not know if the same cable works with a VSD-A.