The big fuse, most likely in the fuse box under the bonnet.
The vectra has 3 fuse boxes, 1 under bonnet, 1 in passenger side, 1 in boot.
Also, check the wiring onto your starter and alternator, and the main earth from battery and engine to the body, if 1 of these is corroded / loose, you'd get the same simptoms.
Wayne, January 2009