Your statement, "Vibration is normal at low load - when Automatic Recirculation Valve (ARV) is close and at full load - when ARV is fully open", is confusing. Let's change this discussion from "load" to "flow" and show us what this means on the pump's head curve which will be much more useful and informative.
It would also be helpful to the members here if you can provide background information (e.g. Has this issue always been a problem? <--suggesting a design issue OR something that recently changed <----suggesting a change in conditions (flow demand, NPSH issues, new piping lines, etc.) .
Finally, do you have spectral plot to provide? showing the difference between normal flow/load vs partial flow/load
Post-Edit : as I re-read this, my understanding now is that only at what you call "Partial Load" seems to make more sense. I would still request you provide operating conditions "flow/pressure" on the head curve. Also, is it possible that some of the high vibration may be coming from the recirc line??? have you taken data from this valve/line during the instances of higher vibration?