I used to have a Vectra as a company car and it did the same thing. I noticed that the interior light and one tail light remained on all the time.
I cannot remember the precise cause of the problem but the dealer who fixed it said it was something like a 'contactor unit', or possible a 'power timer' or something like that. He also said that they had seen a few fail, (mine was an N reg car).
Therefore, I suggest you have a word with your local Vauxhall dealer or specialist as it might save you a lot of bother in the long run. Also, do not be too upset about the battery, at 5 years it was pretty well at the end of its life anyway.
Lee
December 2007