OK, I double-checked in the service manual and there is a redundant safety system I'd forgotten about. My description above is correct, so far as it goes, and the vacuum line under the hood that he was asking about is the one that would normally go to the distribution manifold on the firewall and provide vacuum to the speed control actuator.
However, there is also a mechanical vacuum dump valve on the brake pedal as a fail-safe. The brake light switch should deactivate the speed control electronically (as does a clutch switch), but in case that were to fail for some reason, there's also a mechanical vacuum dump valve on the pedal that will release the vacuum from the speed control actuator servo motor, mechanically releasing the throttle regardless of what the speed control computer is telling the actuator to do. That's the second vacuum line you saw going into the car.