Current Process of writing a VIN does not work with simple did write that VCM Scanner does. Here is direction on how to write a VIN on the new controller that doesn't have VIN written. This feedback should go back to HP Tuner as well and they can make changes for next release.
Before running a command to write a VIN it is necessary to do code clear first. During same key cycle subsequently you can rewrite a VIN. Attempting to write a VIN before executing code clear will not work.
Step by Step Process.
1. Connect your scanner to your ECM (same as connect to vehicle)
2. Go to Diagnostic and Info (yellow engine icon) and select clear codes
3. Go to Controls and Special Functions (green icon)>>System
4. Click on Change VIN >> Type in 17 digit VIN and click OK
5. Your VIN is now written. Key off and let ECM go to sleep.
6. In the VCM Scanner Disconnect from vehicle.
7. Key on(power on)>> Connect to vehicle and VIN you wrote will be read from the controller
BTW you don't need that CGM. ECM and BCM are on the same CAN bus therefore no gating happens between the them. Not sure what is the purpose for the BCM in your setup? Your DLC can go directly on that bus but make sure it is 60ohm terminated bus.
Fire your dealership Tech, he has no clue what he is talking about.