Hi Teebee and welcome to the site. Please update your profile with your information regarding where you are from and the model details of your car, engine size, etc

In answer to your questions
- There is no need to fit Vectra camcovers to an Omega - none at all. The main reasons gaskets fail on the existing covers is because -
- Ham fisted Mechanics overtightened the bolts - they only need 8nm
- Non genuine gaskets have been used (essential to use genuine)
- The breather system needs cleaning (Blocked breathers cause camcover gasket failure).
- Incorrect sealent has been used (important to use GM black goo)
Not a bad job to do yourself if you are reasonably confident.
As far as the misfire - if you have new plugs and leads, and no engine management light on, then the DIS is a likely cause. I have one here, and once I know your model and if it will fit, you can "borrow" it for a test, before you buy a new one, to see if it cures it. If not, just post it back, if so, you can either send it back and buy yourself a new one, or keep it for a small cost.
EGR - they can be cleaned - but were you getting any EGR related codes? You could just blank it off for now, that would eliminate any trouble from it ;)
As far as the headgaskets go, failures are very rare. Were you getting any symptoms such as - fluids mixing, overheating, or over pressurising of the coolant? If not, I probably wouldn't worry.
You can get a dye to put in the coolant which changes colour if it contacts exhaust gasses, this is probably a more reliable way.
Hope this helps?
Let us know where you from or if you need anymore information.
Cheers,
James
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