Know your audience. If you are interviewing to work for a company that encourages a plant based diet and steers clear of the meat and dairy industry, you might want to learn a thing or two about their values. Depending on which direction you sway, you can answer fairly and diplomatically along these lines:

"The vegan lifestyle is great for some individuals. I have had clients achieve excellent results by cutting out meat and dairy. I also know that for those who tend towards anemia may need to get their protein from other sources like eggs or cheese. I don't think being vegan is overrated, but I do think it's important to consider someone's health history before recommending any restrictive diet."