Why would the VSS effect the tach? The VSS is for the speedometer.
I would remove and clean the tach sensor mounted in the IP Gear cover near the oil fill. It is the large ~1" nut with the 2 wires coming out of it. If cleaning it doesn't work, you may have to replace it. Do you know someone else with an IDI that you could possibly borrow a known good sensor to verify, since a replacement is ~$60+......
Now, if the speedometer was bouncing, or the E4OD (if equipped) were in limp mode, I would check the VSS..... but not for just the Tach bouncing.......
Or if you have, or know someone with a laser tach you could also verify if the engine RPM is really staying steady, if it sounds like it is, it more than likely is, but is another option for verification......