Think of veganism as a religion. Some vegans may think they're eating the healthiest diet available, but I don't think any went vegan JUST for the health aspect. The most important reason for going vegan is ethical: do not hurt animals. A vegan can be incredibly unhealthy, KNOW their poor health is down to their unnatural diet and would rather use an artificial supplement than risk harming an animal. Almost every one of them is of the philosophy that "humans and animals are equal, but humans are thinking animals who can make supplements, so we shouldn't hurt other animals".
Some are happy to have their belief and follow it on their own, some think they're as healthy as it gets, some think they aren't 100% healthy, but that they've found the "happy middle-ground" between health and ethics, some supplement, some do not, some may want to talk to you about it, in case you're interested. Most vegans are courteous and respectful and, when told you aren't interested, back down. But there are some fringe lunatics who, like any religious zealot, want to "prove" that their way is the only way and "convert" all us "sinners". Just as enough demand can get evolution taught as "just a theory" in some creationist Christian communities, enough vegans in medicine (or just lobbying or funding) can get studies published saying veganism is the only natural way for humans to live.
And, like any religion: if you're not interested, ignore them.