Yeah, that much of a vacuum leak would make the engine run pretty crappy. If the vacuum line is ran directly from the carb to the climate controls and one of the other lines from the controls to the diaphragms is missing or leaking (or a diaphragm is bad), whenever he messes with the climate controls it could open that line and cause a massive vacuum leak that would kill the engine. Just some thoughts.
Yeah, that makes no sense to me. How does the system get any vacuum at all if the large hose from the carburetor to the canister isn't connected? Something may be connected wrong, but we don't have anywhere near enough information to see if that is the case.