Really it depends how much you need a reliable car. By the sounds of it your Vectra is worth sod all, and you'd be looking to pass it on for a couple of hundred quid. So....unless it would cause you serious problems to break down in the middle of nowhere and/or on your way to work, then just keep it until it breaks TBH. So what if it needs a cambelt? It might snap tomorrow, or it might last thousands more miles. If the roof it rusting- just paint some Kurust or similar on it.
My Mondeo cost me £550, bought off my father in law with 4 months tax. I got £350 from my own company when one of our drivers drove into it and broke the rear bumper; my mother and I have since done at least 2000 miles in it with the bumper held on with gaffa tape (though declaring the knock put my insurance up by 40 quid ). The car owes me less than nothing really, it's due its MOT net week and may well pass, in which case I'll just keep it till it breaks; if not I'll strip it and sell all the glass, lights etc. on ebay before I get someone to lift it away for scrap steel (which I'm sure they'll do for free, it's heavy enough).