Changing Options of Underloaded 160kw Motor?

It would seem to be over rated, but does it have any peak loads it has to handle?

no peak loads. Main motive is energy saving by smaller rating motor.

But then why did they select a 160kW motor? Cushioning?

Motors use only what they need to use, plus losses inside the motor. All you will save is the very slight difference in the losses, maybe a fraction of a percent of total consumption. The cost of the new motor must be compared to the energy savings in order to properly evaluate this. If it runs 24/7/365 it may be worth it, but if it runs intermittently, as vacuum pumps often do, the payback may be too long to be of immediate impact.

yes its running 24/7/365

The smaller motor will run hotter and not save any energy.

They say at about 80% of the rated load the efficiency is maximum,so select accordingly

In India at 415 V (how did you manage 426V?) 3 phase

The power consumption is sqrt(3)x415x90x0.85 = 55KW and even at 426V(?) is 56KW

At 165KW you are running the motor at approximately 33% Load. At this load though the efficiency could be a few percentage lower but I won't have expected a high 0.85 pf. A pf of 0.65 to 0.7 would have been more probable.

The basic question is still why this very highty overrated motor?

Though I am not much aware of the vacuum pump characteristic curves, may be under certain load conditions it might need that power.