There's a difference between an unsolicited shipment and a solicited, but erroneous, shipment
In the USPS recommended actions, there is. In federal law, there is not. The legal definition of unordered merchandise is "merchandise mailed without the prior expressed request or consent of the recipient" (See USC 39 3009(d)). The people ordered some merchandise but what they received was something else that was mailed without prior expressed request or consent of the recipient.
IMO, returning the Vita is the moral and mannerly thing to do, and the company should pay for shipping and perhaps even compensate the recipient for their time to return it (maybe a free game or something). But per US law, the recipient has no obligation to return it. Of course, as noted, this incident took place in the UK and under UK law, with which I'm unfamiliar.