1987 420 Sel idles ok stop, but leaving from dead stop it tends to misfire slightly(especially if I press the gas pedal a good amount) but then evens out as I back off of the accelerator. I recently replaced the plugs with Bosch Platinum+2's ,new rotor(old one had pitting on one side), and added fuel injection cleaner. Also found a vacuum line off on the throttlebodly base. It idled better(smoother)but still stumbles at the beginning of accelerating until I back off slightly. Could the timing be set wrongly because of the idle vacuum that was off and not put back on by the previous mechanics who supposedly adjusted and replaced the plugs and timing to idle as smooth as they could get it?

Wrong plugs get Bosch F8DC4's as the others are not hot enough to run in the MB's and they are non-resistor plugs. What you put in are resistor plugs. I would also suggest replacing the coil as it provides the spark. I would replace the fuel filter and check valve to ensure proper pressure to fuel manifold for injectors and dont use additives as they can cause bigger chuncks to clog the injectors all at once.

If you replaced the rotor you should have also replaced the cap. If one is bad so is the other.If the wires are old they are probably way old. Most of your issues sound tune up related for sure.

Injectors could be an issue, You could rebuild them pretty easy. Go to benzworld.org or peachparts.com to look into that.

Lots of what ifs but a real replacement of old parts is the best place to start.