Irmscher don't list a rear bumper for the Vectra C Estate, only bumper protection strips. (They do list a front lower spolier).
Vauxhall lists OPC, VXR, Line Pack 2 options:
Extension, Rear Spoiler Lip, Rear Bumper, Primer (Line Pack 1, Line Pack 2) (If required can also supply 93165025 - VXR Styling glue kit)
GM Part # 93199105
Moulding, Rear Bumper Panel, Primer (OPC, VXR)
GM Part # 9271538
Edit:
Also
Extension, Rear Bumper (Except Parking Pilot System)
GM Part # 13100340
Extension, Rear Bumper (With Parking Pilot System)
GM Part # 13100341