You ahouldn't need to map anything if the attribute in Windchill has the same name as the parameter in Pro/E.
Also I think you need to go one more level down from EPM Document:
Type Manager:
EPM Document > Workgroup Manager CAD Document > Edit (button)
Template (tab) > Highlight "Attribute Root" > Add Attribute > VENDOR > Save > OK > Check in Workgroup Manager CAD Document
Mike Foster
ATK