When evaluating vendors/suppliers, what "green flag" behaviors do you look for as indicators of a strong future partnership?

For me this is about how interested they are in your business and future strategy not just about the tech they are selling, On top of that it is an open and transparent approach, ability to work with you to drive your strategy and an aligned view on values. As an example we will not work with anyone who does not have a full sustainability report.

For me the top three are:
Open collaboration on future goals / strategic fit
Thought Leadership 
Alignment on Values 

Few of green flag behaviors I see in Suppliers are: 1. Commitment towards sustainability & ESG goals, 2. A strong quality and engineering leadership involvement, 3. Investment in worker training both internally and externally, 4. Strong engineering and technical documentation, 5. Minimal management interference in quality and operations decisions, 6. Thorough management of both tier 1 and tier 2, including regular audits, 7. Internal audits both technical and non-technical brings the reveal shortcomings & they are addressed promptly. 

Common goals
Strong level’s of transparent behaviour that creates trust
Fair agreements that appropriately share risks and rewards

Minimum and reasonable contract redline; trying to understand client's needs and tell them what is actually not needed; commit to providing A-team and provide their CVs; creative in structuring commercial deals.