It is DIY-able but it's not a job for a beginner and you need a pretty comprehensive tool kit, including engine support beam, clutch aligner, sturdy (not weedy argos, halfords cheapo) chassis stands, trolley jack etc.
You have to remove the front suspension and subframe just to get at it.
Theres too much info to type in, get the Haynes Vauxhall Omega manual, it has instructions and pics for the V6 which is the same as the Vectra.
The Vectra manual does not cover the V6.
jjcale, August 2009