Do you feel any pressure on your hand when you have it over the filler hole? If your car has a lot of miles, I am wondering if maybe the rings are worn to the point where you have excessive blow-by?
I think the vacuum hoses all have their source at the intake manifold. For most hoses, disconnect the hose at the intake manifold, cover the hole, and see if the car runs better. If it does, look down that vacuum line for a leak. For the Fuel pressure regulator vacuum line, shut the car off and pull the line off the regulator. Check if it is wet with or smells like gasoline. That would indicate a bad FPR.
See my previous post for how to check for leaks at gaskets.
I'll try to post the FSM page for '92 L27 rough idle for you.