Is it sufficient to maintain patients with atrial fibrillation and established PAD on a DOAC or VKA alone, or is there an additional benefit to adding an antiplatelet agent for CVD benefit?  

If they have medically managed PAD with no recent intervention/revascularization, anticoagulation alone should be sufficient especially if their bleeding risk is not low.  In general, most patients on anticoagulation for AF do not need to also be on anti-platelet agents for secondary preventio...