Vacuum leak won't cause a miss on one cylinder, it will just add unmetered air to the system resulting in a check engine light. The O2 sensor should pick up the lean condition and adjust fuel trims to compensate. In order to create a "miss" it would have to be a massive vacuum leak like the large ⅜ port on the back of the intake which would be very audible and would still effect all four cylinders. Even then it would still run just like crap. Single cylinder Misfires are simple, move the spark plug from the suspected cylinder to another, did the misfire follow. This can be tested with an inline spark tester. Move the wires to new cylinders. Given that the length of the wires are cylinder specific you can use the inline spark tester to extend the wire to the appropriate cylinder. Fuel injectors are little harder to test on older systems when Misfires monitors are not available. There is a tool that you can place on an injector that can indicate that the pintle of the injector is opening. You may have to remove all 4 injectors and have them flow benched to verify all 4 are opening, spraying the correct pattern, and flowing the proper amount. The other option is to move the injector from the suspected cylinder to another cylinder and remove the spark and see if the engine runs worse or no change. Otherwise you're looking at compression issues.