What is a vendor security questionnaire?

It is the form a larger customer sends before they will buy from you, asking how you protect their information:

• It is a sales document now - failing it loses the contract, so it is a commercial issue rather than a technical one.
• The questions are predictable - multi-factor authentication, backups, patching, access control, incident response, and who your own suppliers are.
• Honest answers beat impressive ones - a claim you cannot evidence is worse than a gap you are fixing.

Prepare the answers once and reuse them. Most of what it asks about you should be doing anyway.