Adding open captions automatically is not currently supported in Vyond Studio; however, there is an option to add a text box at the bottom of each scene and type in text to mimic open captions. Enter and exit effects are also applicable for the text box.
1) Select a scene for captioning in the timeline
2) Select the Text icon in the Asset Library
3) Select Caption in the text box options panel and type in text
Below is a video example of captioning: