Your going the correct way , vac supply to solenoids will come from brakes vac pump or on the pipe that goes to the brakes servo .... somewhere there anyhow .
With vac guage at the brakes vac pump take off point I reckon 25hg of mercury sounds good .
Trace the pipe from egr valve to the control solenoid ...... test the vac at that point , if your vac is down at that point you are loosing it somewhere ..... if its the same as the vac pump reading I would suspect the egr control solenoid is leaking the vac off .
I'm assuming here but things using vaccum ..... Egr , swirl flap control, is inlet manifold strangulation flap electric or vac control ?? Not sure but don't think it has turbo intercooler bypass ( dozer valves ) it does have egr cooler bypass so all these just need ruling out that they not loosing vaccum
Turbo vnt mechanism ..... vac or electric? .... vac system normallly have a vac resvoir aswell .
I have a superb bit of kit at work for vac leaks or boost leaks ....smoke pro machine , just smoke it up from brakes vaccum take off and it will fill the whole vac system ..... just look for the smoke .
Keep at it your close to full fix I think 👍👍
