Rathbod's not wrong Barchetta, whatever it states in the service book!
That certainly used to be the case but manufacturers CANNOT refuse to honour a warranty just because the car wasn't serviced by a VW dealer during its warranty; that would be illegal.
However, the parts used by an independent must be either genuine VW parts or parts of a similar quality otherwise the warranty CAN be voided, and the workmanship must also be up to the same standard.
Hence it is a risk going to a non-franchised dealer, but if you have confidence in one (and I know of at least one around here who is just as good as the Main Dealer AND uses genuine VW parts only) then there is no reason whatsoever why you shouldn't use him rather than pay the high VW stealer prices!
Having said that, although I use the non-franchised dealer for my (old!) Polo runabout I only use the Main Dealer for my Eos! So I don't take the risk, and I must say they have been extremely helpful so I don't want to risk upsetting them!