First off, how are you so sure that Vader is not using the Force when he corrects his joystick? The Force is supposed to be intuition and philosophy. Does there need to be bolts of lightning for you to know he is using the Force?
The reason Luke disconnects his targeting computer I assume would be the same reason he trains blindfolded against the training droid aboard the Millennium Falcon - to completely immerse himself in the Force.
For him it's a new concept, for Vader it's old.
But let's assume that joystick = no Force involved:
"The Force is strong with this one".
Maybe Luke's potential as a Jedi countered Vader's space wizardry, maybe he intuitively threw off his aim?
Also, maybe something in Vader stopped him from killing the, for him, unknown X-Wing pilot?
He certainly didn't have a problem blasting the other pilots.
We know that Vader eventually redeemed himself by saving his son, which he was allowed to do because Luke refused to cut him down.
Vader not killing Luke here is yet another example of how a Jedi's passive attitude could influence more than the Sith's direct.