Servicing requirements are outlined in the Approved Used warranty booklet on page 11 (copy below). The requirement as outlined in the warranty booklet states that the car should be serviced in accordance with VW’s recommendations; it does not state that it has to be serviced by a VW dealer, although there should be less risk of a legitimate warranty claim being denied if the car has been serviced by VW. Therefore you can get your car serviced at a VAT registered non VW dealer, provided it’s serviced in accordance with VW’s recommendations - i.e. all work that forms part of the service is competed and genuine VW parts / fluids meeting VW spec are used.
If using an Indy, then ensure the invoice gives comprehensive details of work carried out, including part numbers of parts / fluids used to minimise the risk of VW declining any future warranty claim.