Figured this out for my Bush Mule. Didn't want it on the Rudder, like some of the YouTube demos.
I wanted the differential controlled by the VRA knob on my TNY i10.
In the end it was very easy to do. The i10 is easy to program. I just needed two of the linear Mixes.
My Throttles are on Channel 3 as normal and on Channel 7.
Mix 1: Master is VRA, Slave is CH3, Low side is +100% and High side is also +100%, Offset=0. It is switched On by SWD.
Mix 2: Master is VRA, Slave is CH7, Low side is -100% and High side is also -100%, Offset=0. It is switched On by SWD.
And that's it.