What year?
With the vac line connected, at idle you get a min fuel pressure........ when you disconnect it, fuel pressure should increase 5-10 psi......
Vac in line at idle should be around 18", with a gage.......
You have a misfire, so you need to find out which cylinder is misfiring......you have coil on plug setup.......one at a time, disconnect the harness to each coil.....there should be a noticeable drop in rpm........if there isn't, that is the cylinder with a problem.......
You put in new plugs....were they OEM? If yes, once you found the cylinder, that is misfiring, swap a coil with another cylinder.....if the misfire moves to the swapped cylinder, you need a coil.....if the misfire stays at the same cylinder, then try swapping a plug...if the misfire still stays at the same cylinder, you need to check compression.....you also need to check for a pulse signal from the PCM.....if all this checks out, you have a possible injector problem....