It looks like the assumption is that folder specified via .yml output_dir is relative to package when using vt_validate_source e.g.
> vt_validate_source()
Error: Error during validation of package. Error:
* callr subprocess failed: Error in rendering validation report Error: The directory 'C:/Users/mvendett/Documents/Github/RPackageValidationTutorial.04E02/C:/Users/Public/R_validation/validation' does not not exist.
It looks like the assumption is that folder specified via .yml output_dir is relative to package when using vt_validate_source e.g.