Over the past decade, voice user interface (VUI) design has been steadily growing, along with a growing VUI presence in consumer markets. However, there is currently a lack of widely-established guidelines for VUI design. While many sets of VUI guidelines have been proposed, they tend to be developed independently of each other, leading to a lack of consensus on appropriate guidelines for VUI design. This can hinder the wider adoption of practical VUI guidelines. To address this gap, we performed a large-scale meta-analysis of 336 VUI design guidelines that have been proposed in academic literature. Using thematic analysis, we present a unified and synthesized set of 14 guidelines, representing the most universally proposed principles of VUI design as captured by the 336 VUI guidelines identified in academic literature. We hope that this synthesized set can address several of the challenges to the adoption of VUI guidelines in design practice.