This paper presents a novel Vietnamese speech-based question answering system QA-CarManual that enables users to ask car-manual-related questions (e.g. how to properly operate devices and/or utilities within a car). Given a car manual written in Vietnamese as the main knowledge base, we develop QA-CarManual as a lightweight, real-time and interactive system that integrates state-of-the-art technologies in language and speech processing to (i) understand and interact with users via speech commands and (ii) automatically query the knowledge base and return answers in both forms of text and speech as well as visualization. To our best knowledge, QA-CarManual is the first Vietnamese question answering system that interacts with users via speech inputs and outputs. We perform a human evaluation to assess the quality of our QA-CarManual system and obtain promising results.