Im not exactly sure what you mean, are you trying to get the VG248 and 7914 to function together???
When setting up a VG248 user you set them up as a VGC phone, you dont actually register the device anywhere, thats all configured in the 248 itself.
The 7914 is selectable when you've setup your 7960 phone, its listed as an expansion module in the phone's settings on CCMAdmin.