A bit of an odd one, as doesn't your YouTube subscription allow 10 devices?

Anyway, what you need to do is sign out from the Virgin Media version.

Easiest way to do this is to go into the app, go to the top where there is the symbol for your account. Scroll over to the larger symbol and underneath it [or one if there is more than one account there] you will see a small head and cog symbol. Click on that and it will let you sign out.

Alternatively, go to settings at the bottom. Click that and go down to reset app.

If you don't think you are anywhere near the 10 devices, you can see where you are signed in via your google account - go to security and then manage all devices. Be aware that the names there are not all explicit, and you are probably signed in to the same devices more than once. Remove anything marked as inactive, anything doubled up, and any device you are obviously not using, e.g. an old phone.

The 10-device limit has nothing to do with VM as its boxes are for streaming YouTube and not for downloading (which is for mobile devices and PCs). And even if you try to exceed that limit YouTube automatically de-authorises the oldest device in your download list - so not a big issue.

As you're leaving VM it would be good practice anyway to sign out of your Google account on any device (like a V6, 360 or Stream) you no longer control. Also sign out of any other apps like Netflix.