Recently, virtual personal assistant (VPA) applications have been employed in mobile devices, which provide a natural and convenient interface between human and machines. As the VPA services become popular, consumers demand for a wider service than their scope, so the rapid development becomes more important. This paper introduces a dialog act modeling approach for VPA applications, which is an extension of a Latent Dirichlet Allocation model. This approach enables the rapid and cost-effective development by reducing human efforts for manual labeling and the development of a fail-safe product by incorporating domain knowledge such as dictionaries, cross-lingual data, and logic rules. The experimental results showed that a reliable and high-performance dialog act model was built only with a small volume of labeled data and domain knowledge.