In vpc go to edit and select "windows xp home settings". Select the 'com port 1 setting'. You will see modem in the drop down menu. Hit the drop down menu and see if your usb (driver) is there, if so, select it and hit ok. Vpc may "unexpectedly quit" which was a good thing for me, anyways. Restart vpc with usb cable attached and it should notice the phone.
If it works remember to always have the phone attached before starting vpc. I would also always 'save all and quit' so you don't have to go through the process again.
I have a Nextel phone also, i730. I also have a xp box so I never tried it under vpc. I just tried it in vpc and it sees the usb connection as 'usbmodem' but I have not installed the iDen update app in vpc. Let me know if it works.