Considering the amount of interaction with vendors outside of contract award, what non-contractual agreements or documents are typically used in conjunction with contractual agreements to indicate expectations for the contractor-vendor relationship? (i.e. statements of understanding, legal requirements to do business with DoD)
The answer to your question can be found in FAR Subpart 16.7 which covers Agreements. A basic agreement, which is covered in FAR 16.702, sounds like what you're describing -- "(a) Description. A basic agreement is a written instrument of understanding, negotiated between an agency or contracting activity and a contractor, that (1) contains contract clauses applying to future contracts between the parties during its term and (2) contemplates separate future contracts that will incorporate by reference or attachment the required and applicable clauses agreed upon in the basic agreement. A basic agreement is not a contract."
The first step to take is read what FAR 16.702 says about basic agreements and decide if it's something that could fit your needs. Next, discuss with your legal counsel what you'd like to include in the agreement, and they will be able to advise you on any "legal requirements" to include in the document being contemplated. Also, your legal advisor may have examples, might even help you write it and, at the very least, will review it for legal sufficiency.