If the vaccum gauge is reading zero you have a leak somewhere. A healthy 4G63 should pull between 15-18in/hg at an idle.

The factory O2 and DP should not pose a restriction for a T25 @ 15psi.

After locating your vaccum leak, you can pull the hoses off of the boost solenoid, but leave it electrically connected. Then put an 1/8" T connector between the boost source and w/g actuator and run the extra line off the T connector to the air canister/MAS. With that you should be able to sustain 15psi, if not you will need a boost controller, but be sure to check and make sure the boost does not go to high. If the boost spikes to high you can lower it by restricting the hose going to the canister. I have used pellets(the type for a BB gun) and drilled a 1/16" hole in it.