I see you are in the USA. FSMA did help us align to some degree on what customers ask for. I would strongly recommend you take a FSMA Preventive Controls Qualified Individual class based on the Food Safety Preventive Controls Alliance format. What you ask of your suppliers should be targeted collect what you provide customers, unless you are running your own quality program and generating your own data.
Here is a list I would prepare:
Product specification and Technical specification (when appropriate)
Continuing guarantee to meet regulatory requirements
Country of Origin
Certificate of analysis showing critical parameters specific to the batch
Most recent GFSI report with corrective actions implemented
General Process flow diagram indicating hazards controls (microbial, foreign material, chemical, etc.)
List of programs maintained by the supplier with short description such as:
Training
Environmental Monitoring
Pesticide testing
Foreign material control (screens, magnets, metal detection, X-ray)
Microbial control - Add a Validation overview for any micro reduction steps or critical control points
Food Defense
Intentional adulteration
I see in general frozen vegetables are at risk for pathgenic E.coli, Salmonella, and L. monocytogenes. Most of the chemical hazards are vegetable specific, and the physical hazards are going to be control of food contamination from materials that exist from the farm through processing including items such as cutters, conveyors, etc. You want to make sure you have programs to address what the FDA has published as a risk to your food.
I hope this helps you get started.