As @[deleted] says the downpipe, flexi pipe and catalyst are all one piece on the Vectra.
Now then, are you all sitting down!
A genuine GM Part - Pipe, Exhaust, Front, with Catalyst (Ident FK) - part # 93189884 (supersedes 24461058, 24434669) costs £1054.00 (inc surcharge of £80.00) :jawdrop:
However, they are well manufactured and good for longevity.
Note: The genuine ones have the pre-cat fitted in the top of downpipe where it meets the manifold.