Siri on Volkswagen RCD 310 using Bluetooth
Found a reliable yet cumbersome workaround to have Siri play over the RCD 310 stereo using Bluetooth... 


Using your car's steering wheel control, disconnect the Bluetooth link to your phone.
Open Siri by holding your iPhone's home button. 
Re-activate the Bluetooth connection from your car to your phone.


Voilà, after a few seconds you can have a dialog with Siri in your car... at least until you leave the Siri screen.