Vacuum leak.

1993 Cadillac DeVille 4.9 mileage: 180,000. I recently acquired a Cadillac from a friend. It’s a nice car, but the catch is it will run but it barely idles. If you step on the gas the engine dies. He gave up on it after a guy he knew “fixed” it for cheap. They had a sharp dispute and parted company on bad terms. My friend shelled out several hundred dollars for a car that was useless to him. This supposedly knowledgeable car person, who isn’t part of any auto repair outfit, offered to put new intake manifold gaskets on to fix a coolant leak. That doesn’t quite make sense to me. What does the intake manifold have to do with the cooling system? This is my first Cadillac, so maybe there’s something I don’t know yet. Also my friend said the car ran perfectly before this guy got his hands on it. I checked and re-checked all the vacuum connections around the throttle body and followed the lines to their connections. Everything was fine. I hooked up a vacuum gauge and fuel pressure gauge. The fuel pressure was fine, but the vacuum gauge barely registered any vacuum while idling. I made sure the gauge was working by hooking it up to another automobile, and it was. So it seems to me that the guy must have messed something up putting the manifold back on. My question is if there is any way of isolating the vacuum leak, or do I just have to take the intake manifold off and replace the gaskets again? And am I right to assume it’s a leak somewhere around the manifold? Or is there any other area I should check? The guy also was supposed to put valve cover gaskets on and change spark plugs. I found three new gaskets in the trunk. By looking at a parts supplier online I see that two of them would be from the intake manifold gasket kit. One is squareish, about 3” by 3” with three bolt holes. The other is round with a projection, and has nine bolt holes. The last gasket looks like it comes from a valve cover gasket kit. It has eight circular holes, and four bolt holes. I’m not sure what these gaskets are for until I start taking things apart. I’m not sure why he didn’t put these in. Anyway, any suggestions about if I’m on the right track would be appreciated. I don’t want to tear things apart unless absolutely necessary.