Usually the rear bushes that require replacing first on the Vectra C and Signum are the upper control arm (13) outer bushes, as that's what was causing a similar issue on my mates Vectra C. Vauxhall don't actually list the bushes as a replacement part, so if purchasing from the dealers they will sell you a new arm complete with bushes.
Powerflex make them, but there are two types for the upper control arm outer bush so make sure you get the correct one. It's also prudent to renew all the nuts and bolts as they are 'pinch' bolts.
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