Hi,
the Vr adds variable parallel resistance to the two collector loads.
You only need one side to be adjusted for one polarity of output offset.
If you leave Vr out and measure the output offset you will find it is either +ve or zero or -ve.
Now add 100k from pin7 to pin5. Does the output offset move closer or further from zero? try different resistors. try pin7 to pin1 if the offset is needing correction the other way.
But, wait,
look at the datasheet. the first3 sections of table 1 show low levels of offset and drift. I suspect this wafer has been laser trimmed to meet this specification. If there is output offset, it is more likely due to an input offset. Fix that first before unbalancing the collector loads.
Instead of switches, you could use 0.1inch pins and short them out with a 0.1inch plug. Or delete the switches and use just one output resistor to suit your loading.