Question:

Why does my Vault Loader CSV fail when attempting to set my Yes/No field to TRUE, FALSE, YES, or NO?

Answer:

This is a known issue within Vault itself.  When attempting to set a Yes/No document field, both with Vault Loader and with the API, users should use 'yes', 'no', 'true, or 'false' in all lowercase.  

Related Documentation:

Vault Help Documentation: Known Issues for Static Reference Constraints