Vacuum Contactor Wiring Diagrams
What is the purpose of this resistor and capacitor


After full activation, C raises impedance with low loss, R lowers Q to avoid LC  resonance.


  potentiometer in series? 



too lossy (hot) for holding current.



  Don't they each need their required VA? 



No.   S is rated for  Start, R when added to S reduces to holding power for Run operation. They dont’ have to be identical coil impedance.



  do they ever risk opening as the DC voltage bounces to 0 V



No. these would shading pole type relays like those that are AC/DC