My experience of aftermarket mirror glass replacements is poor, both with Vauxhalls / Omegas and also with other vehicle makes.They either don't fit properly because the clip retainer ring is the wrong profile or shape, or the heated element terminals are the wrong size and need modifying. Many arent the exact shape as the original either.
The tenner spent on a cheap aftermarket mirror glass is generally a waste of money, in my opinion, especially when it falls off at speed either scratching the door panel if the wires stay connected or smashed on the road if they part company.
I would always buy genuine VX mirror glasses, either new or used. I have never had a problem with used genuine mirror glasses. They fit and look correct, whether they have the heated element symbol on them or not, and are usually cheaper than a tenner.