I keep an old laptop just for the purpose of updating VP50Pro firmware - not that there has been much recently. Make sure you set the baud rate to 57600 on both the VP and the Terraterm pro. I think I had an error like yours until I set the baud rate correctly.
Stuart

861v6, HD621, DSP5200, DSP5500HC, DSP33, ROKU3, Transporter, VP50Pro, Pioneer 6010, Pronto TSU/RFX9600.