Question:
Can document fields that have default security set to Hidden be queried using the Vault API?
Answer:
The setting is found in: Admin --> Document Fields --> [Document Field] --> Security Overrides.
If a Document Field's Default Security is set to Hidden and there are no Security Overrides then the field is not able to be queried using the Vault API.
Related Documentation:
Vault Document Fields Documentation: Types of Field-Level Security