NOX is caused from combustion chamber temps to high. Pulling a vacum hose will want to lean it out and then the PCM will adjust the inj duty cycle by the 02 and adaptive fuel to compensate for it. And if you could get a large enough vacum leak to make it lean I dont think it will help as lean can make comb chamber temps higher.
I agree as its best to have a good tech look at all the sensors on a scanner or data recorder. I would want to know what the adaptive memories are running at as much the the 02 and by checking the downstream 02 reading (if it has one as I did not see what year it is) will tell how the converter is storing oxygen and it sets the goal of the upstream 02. But I did not see if its a 96 or older. If all sensors and adaptive memories all reaing in normal ranges you may just have to replace the cat.

You aready replaced the EGR valve which needs to be working right also. Dont forget if you test the EGR many use EGR valves that use exh system back pressure to let the EGR work so if the exh system is modded the EGR may not work correct. It most likey will have an EGR solenoid also that the PCM controls as it knows when to open the solenoid so it will pass vacum to the EGR valve. I would want to be sure the EGR is working. I have heard some say to put some dry gas in it and it may squeak by. Good luck , Ron