Rich,
Are you sure the VFO tuning capacitor is properly indexed to the dial? It should be full mesh with the dial pointer at the CCW horizontal position.
I have never run into a problem with calibrating any of the Johnson VFO assemblies when the dial/capcitor is indexed properly. Is there any indication that the former owners changed any of the caps in the VFO? If the proper marked values are installed then one of them may have changed value. These caps will have specific temperature coefficient values so replacement is not going to be easy unless you are willing to have an unpredictable drift in frequency-even worse than the norm for these VFO units