I had a look at a vectra 2.6 sri, 2000/2001 (?) with 39k on the clock a while back; it was extremely clean, without so much as a fleck of paint chipped and was immaculate inside. It came with a full dealer service history and a limited part and labour warranty too.
My interest was quashed when I looked at the usual fluids under the bonnet. Whilst there was no sign of any mayo in the oil filler, an investigation into the water expansion tank revealed clean water and a thick creamy paste all over the inside walls of the tank.
Given that I was browsing for a prospective motor and not there to buy, I was not all that bothered to walk away from that one. It was either a head gasket problem or perhaps something to do with the oil cooler.
I later confirmed this with some friendly mechanics at my nearest vauxhall dealership. Their thoughts were that 'whatever way I looked at it it could have been expensive and that was a problem they were used to seeing'

oil cooler replacement - £120.00 part then a couple of hundred quid labour.
head gasket - ~£1200.00 all in!

let alone what it might have cost to fix
both of the above at a main dealer. It doesn't bear thinking about.
In hindsight I'm glad I didn't choose to buy a vectra, though I don't find them all that unpleasant to drive. Be wary, if it looks too good to be true it usually
is and there are certainly plenty of other cars out there to choose from, vauxhalls or other wise.