How to use PIC18 with MCP USB Chip to do HID?

[towlerg]

Am I missing anything?

Simple answer, yes.

The MCP2200 is a preprogrammed PIC, which appears to the world as a USB to Serial convertor. Ading a mouse descriptor and the necesary software to service the descriptor is essentially impossible (no flames please). You would be much better of starting with a 18F14K50, modifying a HID sample.

The data sheet gives details of how to change it's PID (among other things). From the data sheet -

The configuration utility provided by Microchip allows the user to configure the MCP2200 to custom defaults. The configuration utility (shown in Figure 2-1) connects
to the device’s HID interface, where all of the configurable features can be set.

.I don't want to use the onboard USB as it doesn't have a vendor id

Incorrect. The VID and PID are changed in the descriptor file which will be part of the compiled project.

eg The portion of gen_desc.bas which defines the VID (among other things), in this case 04D8 hex.

DeviceDescriptor
	retlw	(EndDeviceDescriptor-DeviceDescriptor)/2	; Size of this descriptor in bytes
	retlw	USB_DESCRIPTOR_DEVICE	; DEVICE descriptor type
	retlw	0x10		; USB Spec Release Number in BCD format - 1.10 (low byte)
	retlw	0x01		; (high byte)
	retlw	0x00		; Class Code
	retlw	0x00		; Subclass code
	retlw	0x00		; Protocol code
	retlw	USB_EP0_BUFF_SIZE	; Max packet size for EP0
	retlw	0xD8		; Vendor ID - 0x04D8 is Microchip Vendor ID (low byte)
	retlw	0x04		; (high byte)
	retlw	0x0C		; Product ID: PICDEM FS USB (DEMO Mode) (low byte)
	retlw	0x00		; (high byte)
	retlw	0x00		; Device release number in BCD format (low byte)
	retlw	0x00		; (high byte)
	retlw	0x01		; Manufacturer string index
	retlw	0x02		; Product string index
	retlw	0x00		; Device serial number string index
	retlw	NUM_CONFIGURATIONS	; Number of possible configurations
EndDeviceDescriptor

I don't understand your references to mice?