not that I want to drag you away from your plan but it seems that building a correctly working VNT controller is a big project.
Did you think about a ball bearing turbo i.e. BorgWarner EFR series?
For my SD I modified a GT25 VNT to fit to the flange but finally failed controlling it.
On my 603a I still run the stock turbo but with bigger element IP. There is no turbo lag any more with powerful low end torque.
I am in progress to upgrade with a ball bearing turbo (BorgWarner off V12 AMG, one bank). No idea how this will run but for sure I don't need a VNT.

What I know about VNT control:

input:
- pedal position for setting the desired boost
- actual boot signal

output:
- pwm signal to control a vacuum valve (proportional valve)

Let me know and I'll search on my computer what I collected about VNT controller.

Tom