There isn't a speed sensor on the platter. The stroboscope markings on the platter are for the user to fine tune the speed using the speed adjustment pots. They aren't used to control the motor speed. The servomotor speed is controlled by a feedback loop involving the motor itself and external wiring from a signal generated by the motor, through the speed adjustment potentiometers, and a switch, then back to the control input of the motor. From your symptoms I would suggest examining the wiring and connectors involved in this. I downloaded the schematic from Vinylengine.com. If you have it, follow the path from the block on the schematic called "T/T Motor" (turntable motor) from the VR terminal (white wire) through connector 6 then connector 4 and the speed adjustment pots (VR103 and VR104), back to the switch (IC 105) then to the master speed adjustment pot (VR105) then finally back through connector 6 to the control input of the motor marked "C" on the schematic (gray wire). If you have a voltmeter you can measure the voltage along that path and see if it disappears at some point. Be careful probing on the circuit board. I use needles attached to my voltmeter probes to minimize size. The connectors are the most likely places for problems to occur due to oxidation of the contacts. The master speed adjustment pot could also be a problem but I'd be careful of trying to clean it. If the adjustment is off, then all speeds will be affected. So in cleaning it, you will change the setting and have to reset it. I haven't found a procedure for adjusting it so if you do so, first be sure to measure the resistance so you can get back to the original adjustment. The problem here is that if the pot is dirty and not making good contact, the resistance you measure will be incorrect. That's why I recommend caution on this pot. If push came to shove, I suppose you could set the speed adjustment pots to the middle of their travel and then adjust VR105 to get the speed within range of VR103 and VR104. If I remember correctly, the voltage in the feedback loop is pretty small - less than a volt or so.
Regarding the Vaseline: I doubt that this is the problem. I don't think Vaseline is conductive. If some did get on the board, I would try to use Q-tips and perhaps some contact cleaner to remove it, but I doubt it is the source of the speed problem.