Posting a picture of what your TVs screen looks like would have been helpful.
Attaching a TV box of whatever kind should not cause any problems to the TV.
The TV doesn't necessarily need to be off while you do this.
It should also not be a problem having two boxes or more connected at the same time.
For each device connected to your TV you need to adjust the TVs picture settings for each.
Having set up the picture settings for each device your TV will remember them.
If the Virgin media box is connected by HDMI you need to access the TVs External inputs> HDMI signal format
under Settings and make sure the HDMI being used is set to Enhanced. See below.
https://helpguide.sony.net/tv/gaep2/v1/en/04-09_02.html
What other ways do you watch TV, do you have a conventional aerial attached and if so, what is the picture like?
Is the Sky box still attached, what is the picture like when viewing that.
If you watch TV or movies via USB devices is the picture okay when viewing them.
If using other devices and the picture is fine could it be that the Virgin Media box is faulty.
As your TV is oLED it should have a panel refresh feature within the settings.
I don't know where it will be on your model but on my oLED TV it is under Settings> Display & Sound>
Expert Panel Settings> Panel refresh, you could try something similar.
I take it you have power cycled the TV, switch off, unplug, wait a couple of minutes and restart.
How about a factory reset, before factory resetting you will need to take a note of all your settings and any usernames and passwords you use with your google account and apps.
If none of these things improve your situation then is does look like something has happened,
at any time did you see the installer perhaps touching the screen, applying pressure to these sensitive
screens can cause shadowing to appear and disrupt the pixels.
Did you get them name, contact details and a written and signed written statement from the installer confirming the TV was working perfectly before connecting the Virgin Media box.
If Virgin are still claiming the TV was faulty, you could ask them why would you order a VM box if your TV was already faulty, and ask them what proof do they have it was faulty prior to installation.
Contact Citizens Advice, find out your rights and the correct process for ensuring a resolution
and to get out of your contract without incurring any penalties.
