In my opinion, I wouldn't use VW after the car is out of it's warranty period. Cost per hour of labour is crazy. Plenty of specialists out there with access to all the tools and parts that the dealers have.
I think that as long as OEM VAG parts are used by whoever services the car, and as mentioned, the car is serviced on time - then it shouldn't affect resale value at all, IMHO. A stamp not mentioning VW specifically shouldn't be an issue to most reasonable people when buying an older car.
I guess there could always be those who won't look at anything other than main dealer serviced cars, but think they must have money to burn & unrealistic expectations on an 11-9yr old Edition 30!
My ED30 was serviced by a VW specialist at 40k with OEM VW parts, and VW later sold it to me under their 'Used Approved VW' scheme at 47k with the usual 12months warranty.
I've used APS in Brackley ever since I bought the car - it's a bit more than the round trip of Dan's 60 miles for me, but a few quid on petrol for the honest opinion & experience of those who mostly work with the performance platform cars more often than not is worth it to me.
No mention of VW in their stamp in the book but what does that matter when you have a very detailed invoice to back up a garage stamp?