The correlation between veganism and mental health is a broad topic. I'm sure there is much that has yet to be explored.
But you didn't say much about your background, credentials, or area of focus. What field are you in? Affiliated with a university or other institution that conducts research? If so, which one? What kind of research have you conducted in the past?
To those responding - I would advise against participating in anything until the researcher discloses their identity and the relevant details of the project. Then do your research on them before signing up for it.