1 clean and re-attach the earth connection from the battery to the chassis, and chassis to engine.
2 look for the leak - could be alternator, but it is a vectra so could be elsewhere. Disconnect +ve on battery and measure leakage with multimeter set to mA scale. Expect a few mA. Keep in place and disconnect the alternator, then other circuits till you find it. Nothing will be easy to get to of course, its a Vectra after all.
trover, October 2009