I have this system that uses pump 'x' to deliver fluid 'y' throught my plant. The duty point of the pump is 800gpm @ 179ft.

There is a second, smaller pump in the system, and its design duty point is 100gpm @ 85ft.

The purpose of the second pump is to serve as the main delivery when end user demand decreases. I have been considering using a VFD [on the larger pump] to basically replace this pump. My concern however, is the effect this extreme deviation from the design duty point will have on the pumps.

Can anyone give some advice?

Driver Details:

Pump 1: 460V/100hp/1800rpm
Pump 2: 460V/10hp/1800rpm

In general, a VFD allows you to keep the pump closer to its BEP during demand changes than if there were no VFD. Specifically , when you slow down the pump to match lower demand, you are also lowing the pump BEP. So from this perspective, adding a vfd should improve pump reliability. I think it is particularly true in this case where demand apparently varies as low as 10gpm... so if you are in the 10 -20 - 30 gpm range your 100gpm is way to the left of bep under the existing non-vfd setup

Just to mention some miscellaneous reliability considerations for VFD's"
1 - More likely to hit a resonance during some operating conditions. That may of course reduce your life. There may be strategies for blocking the speed in question if you know about it.
2 - There is more complex equipment driving the machine which is more likely to fail. (a vfd is more complex than a simple breaker or starter).

Need also to check that the allowable operating speed range for the mechanical seal. We have seen seal failures on a VFD pump due to it being run below the minimum operating speed of its N2 gas lubricated mechanical seal.