Recently, usability has become very important in voice user interface systems. In this paper, we have designed and implemented a wizard-of-oz (WOZ) usability testing tool for command-and-control voice user interfaces. We have proposed the VUIDML (Voice User Interface Design Markup Language) to design the usability test scenario of command-and-control voice interfaces in the early design stages. For highly satisfactory voice user interfaces, we have to select highly preferred voice commands and prompts. In VUIDML, we can specify possible prompt candidates. The WOZ usability testing tool can also be used to collect user-preferred voice commands and feedback from real users.