Yes you can fit and use Vectra C, or even later Astra 5x110pcd wheels legally, just make sure you use correct tyre sizes if you "step-up" or "step-down" in rim sizes.
There are good tyre converter sites around, but I prefer these two as they are visual; one from the side elevation and another from a plan view.
Side Elevation:
http://www.kouki.co.uk/utilities/visual-tyre-size-calculatorPlan View:
http://www.wheelsmaster.com/rt_specs.jspHere are some example pictures of Omegas with them fitted; both are facelift cars, but it works equally as well with pre-facelift cars.
Vectra C Sri 19" snowflakes.

Astra 18" Penta wheels

The 1st (blue) car is mine and the silver one is actually an American rebadged Catera which belongs to "CateraMV6".
The offset on the Vectra wheels is different as it is a FWD car (more inward / higher offset - Omega Elite = ET38 vs. Vectra = ET45; so 7mm difference inward for Vectra (FWD) wheels).
I have used Eibach Pro hub-centric spacers. 18mm per front corner and 25mm per rear corner. This is not absolutely necessary, but I think that it helps the wheels to fill the arches better.
Here's a picture from an angle that shows what I mean.