Hi Initiale
Welcome back to the community forums
Sorry to hear of your concerns with cancelling your O2 SIM.
You must be a customer of both Virgin Media and O2 to be entitled to the Volt benefits. If you were to cancel your O2 SIM and there was no other O2 SIM registered to the same address as your Virgin Media service, then this would in turn remove any applicable Volt benefits from both packages.
Though the O2 SIM is billed separately, the SIM is registered to your Virgin Media account to active the Volt benefits. Without an active O2 SIM registered to your Virgin Media account, you wouldn't be eligible for the Volt benefits anymore and these would be removed.