Reading between the lines here, I think the throttle plate in this instance, will most likely serve as to purge the EGR flow.

My thinking is that, if it was intended for a "strangled shut down" then if Martin is correct & it is pulled shut whilst running, then the engine would not even run. Its' function is almost certainly not for the purpose of Vacuum development for Brake Servo operation.

Does the throttle plate have a potentiometer for position feedback?

As an example of one method of operation, a 2.2 Di Frontera will drive the throttle plate almost fully closed at idle & will open it up as greater intake is required. A fault on the Frontera flap system, will generate a relevant DTC. I am not saying that this is the correct MO for the Nissan in discussion.

If the EGR has been blanked off & the intake flap has been forced open & the driveability concern still exists, then I would suspect that the fault lies elsewhere.

What colour is the smoke? Do the revs remain stable when it loses power?

Here is a vac tool that I bought, after someone (who nobody knows ) broke my MityVac! It was about 70 quid + VAT & it works very nicely.