The vault object has the SPFSubDirectoryProperty property to identify the property to use. This is in the format of:
Obj.PropertyName
Obj->RelDef_[12|21].PropertyName
Since the document type property is on the document master and the file is attached to the document version, an edge must be used:
SPFSubDirectoryProperty = Obj->SPFFileComposition_12->SPFRevisionVersions_21->SPFDocumentRevisions_21->SPFClassifiedLevel2_12.Name
This results in directories such as:
.\VaultPath\PID Drawing\
Objects that do not have this property go in the .\VaultPath\ directory.