A fighting force in the SR could not attack a Shard. Shards are large beings of sentient Investiture, much like we are beings of sentient matter. It is true that most of their Investiture coalesces in the SR. However, you can't attack it. If you tried to stab Stormlight, what would it do? Nothing. Investiture may be moved or rearranged, but that would not hurt a Shard as they are nearly entirely fluid, and it doesn't really matter what shape their Investiture is in as long as it is there. The only ways to accomplish this with a large force would be to:
a) Use Investiture to attack it.
b) Slowly siphon Investiture away from it using aluminum, similar to a traditional splintering.
c) Convert all of the Investiture into matter or energy.
These would all be easily counterable by the Shard, and would not bring any advantage having trained soldiers rather than untrained soldiers.
However, I believe the forces Retribution sends out will be to attack the people on the Physical/Cognitive Realms, not in the SR. I can definitely see him taking control of Vorinism, except for the fact that Hoid says he apparently drew his attention away from Roshar and went into hiding, ceasing interaction with planet due to assuming it secured.