2012 vw steering wheel and headunit wont skip or control music after ios 8.4 update.

Same problem here.

iPhone 5 was fine last week on iOS 8.3 - updated to iOS 8.4 and it won't work.

2011 VW Golf with RMS 510 head unit.

I've seen other posts about VW units no longer functioning after the 8.4 update. Unfortunately, this will be up to VW on whether or not they want to fix it by offering a firmware upgrade for their units. Apple issues beta software for developers specifically for this type of reason to allow manufacturers to update their systems prior to public release. You will need to contact VW and try to get them to issue a firmware upgrade for your unit.

Why do VW need to provide a firmware upgrade for their units when the problem is caused by Apple's software changes?

Also, why would customers who own previously perfectly functioning VW/Apple audio equipment need to arrange a visit to a dealer for a firmware upgrade (plus possibly pay for the privilege)?

Because the unit was designed around the IOS software at the time of release and now the software is changed. It's the same thing when the Mac OS or Windows OS changes, the software developers are the ones that need to make sure their product works with the new OS and not Apple or Microsoft to make sure their OS works with the millions of software programs available. This is the same for hardware. Apple isn't going to go to every car manufacturer and verify that their software works with every car unit on the market. The manufacturer is responsible for making the changes to make their unit compatible. Some manufacturers are really good about making these changes and make them easy for the consumer. Other manufacturers say that it worked when you bought the car and they have no obligation to make it work on newer software.

Well there should be an option to revert to previous iOS software, to avoid the customer having to chase around for solutions.