OK, good news and bad news.
1) The "Official Vehicle Dock" does not work properly. You cannot connect the phone mic through it (the 3.5mm socket on the bottom does audio out, not mic-in). Meaning that it is useless if you want to use your phone as a ...er...phone! I'd say that is a big enough "feature" to make it "unsuitable for purpose" in UK law. Unless someone has worked out how to do that or they update the app.
2) You can get the phone to work if you connect it via the top 3.5mm socket. The splitter I bought, when connected to the top socket, correctly directed audio to the car HU and mic-in to an attached microphone. Then you do need the splitter I linked in an earlier comment above plus a mic.
(FYI As soon as anything is connected in the top is disables the bottom 3.5mm socket)
But - here's where I suspect some people are going wrong - the Vlingo software defaults to turning ON speaker phone, so as soon as the call connects the sound is taken from the car speakers to the phone speakers. This may be why some people hear voice commands through the car stereo but then it diverts to phone speakers when the call connects. If you disable this, the sound stays going through the car speakers. Within Vlingo select Menu>Settings>Voice Talk Settings and then un-tick Auto-start Speakerphone. You also need to ensure Driving Mode is enabled in Vlingo.
Happy to try something else if anyone has any suggestions. My suggestion - don't buy the official vehicle dock!
(PS: Have submitted a ticket to My Samsung asking for clarification about the Dock 3.5mm socket carrying mic signal or not).