Thich Nhat Hanh passed away recently (22 January 2022). The influential Buddhist monk, writer, and political refugee from Vietnam was a vegan. Check out this video (2:26 min) from 2014, in which Thich Nhat Hanh explains his views about the reasons for being vegan. Thich Nhat Hanh died at the age of 95 years, due to problems related to a stroke he suffered a few years ago.
At this point, I guess it's my duty to state that for vegans it's important to ensure an adequate intake of vitamin B12 to reduce the risk of stroke (See recommendations here and more ideas on stroke prevention here -- spoiler: try to relax more).
I'm not aware of any explicit mention of veganism in any of his books. But I've come across this Thich Nhat Hanh interview by Oprah Winfrey (the interview is from 2010):"
Oprah: So you wouldn't eat an egg.
Nhat Hanh: No egg, no milk, no cheese. Because we know that mindful eating can help save our planet."