You can't retrieve the Security code with VCDS.
When the unit is put in the car and the code first entered it is stored in the ECU. If for any reason you remove the stereo or battery for any great length of time (more than 2hrs iirc) when the battery etc is plugged back in the ECU sends the code to the unit to unlock it.
If you put it in another car it will not work and ask for the code.
You have 2 choices:
- Take car to a dealer and retrieve the code with proof of ownership
- As Ian says find someone with RNS Manager cable (Only 2 i know are VRStu and Eddie_NL) and they can reset the code for you.
HTH