If there is one, the "GPL contract" in the case would be between Vizio and Linux copyright holders, not Vizio and SFC, since SFC are a third-party in this case.

If there is one, the "GPL contract" in the case would be between Vizio and Linux copyright holders, not Vizio and SFC, since SFC are a third-party in this case. You are indeed right that Vizio can probably get around SFC current suit by arguing that they did not enter into the GPL contract and just deliberately decided to use the code without a copyright license, but that then opens them up to higher damages under the inevitable copyright lawsuit.