Personally I would get as close to OE as possible. I had really bad problems a few years ago when I fitted ET37 and a 3mm spacer on my Cavalier. The OE offset is ET49 on the Cavalier. The car was tramlining like mad and it understeered badly too. I went for several alignment checks and it still felt wrong.
My tyres only lasted 6 months on the front so in the end I sold them on. I still had another 6 months to pay off on them as well.
They were sold to me by South West Tyres in Clevedon. What a bunch of plicks they are. I went back several times and in the end they did the 'gang up and intimidate' thing.
Basically, keep as close as you can to normal. I thought that the Vectra was ET44?
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