... At what point do purchasing agents in the US for both government and private sector look beyond cost and include security as part of the vendor selection process?
When the authorities that provide the money stop insisting that lowest cost is the primary or only valid selection criterion. This 'keep it cheap' approach plagues all government work, and a fair amount of commercial work. Further, it does not damage only security, but has for decades also damaged other aspects of product and service quality. It is the bane of the quality movement (Deming, etc.).