Canbus canbus canbus!!! That's the answer. I don't know why VW brought out various versions of the Polo/Vento. Some are CANBUS enabled, some aren't. I think the first version (RCD 300) wasn't CANBUS enabled. Then came the RCD 310 which was CANBUS enabled. That's why it was easy for people to swap the RCD 310 for the RCD 510. Direct PnP. Now, the Polo/Vento comes with the RCD 320 which, I read somewhere, isn't CANBUS enabled?! I'm not sure though. The more knowledgeable people can clarify that.
But this CANBUS enabled/not-enabled is what is spoiling the whole show. It would have been so much easier if there was just the CANBUS enabled version.
PS: The speakers on the old Polo are much better than the ones on the newer ones. I know because I have both