I agree with Natalie that if you do the Health Coach certification you need to look very carefully into scope of practice regarding how much nutritional advice you can give. Without a degree and licensure in nutrition this advice is likely to be fairly general.
I think this gives you two options. The first is as Karin says to be clear of your own path, but to explain options without demonizing paths that differ from your own.
The other is to market yourself specifically as a vegan. If you are clear before you take on clients that this is the path you walk and the path you teach I think you could build your business around it as a niche. To expand on Karin’s metaphor this would be to say…. if you wish to bicycle I will refer you to excellent bicycle coaches. But my particular skill set lies in snowboarding, and if that is right for you I would love to work with you.