Being a vegan means that you have to more carefully watch your nutritional status. Without animal products in your diet, it is much harder to ensure you are getting all the vitamins and nutrients your body needs. There are plenty of options to make up for this, such as vitamins and minerals and even certain vegan foods.
40% of the US population is vitamin B12 deficient, approximately 2% of the US population are vegans or vegetarians, clearly this is a universal problem and not a vegan or vegetarian problem, more should be done to inform everyone though especially those who are not vegans or vegetarians seeing as how 38% is considerably more then 2%
Being vegan isn't bad for your health, people just don't enough to stay nutritionally sound when they follow a vegan diet. They need to take supplements to make up for anything, including B12, that they aren't getting enough of. Even that deficiency is less dangerous than the heart attacks and other problems that come from animal products.
I would not be so quick to relate the said Vitamin B12 deficiency to veganism. I'm sure there are vegans who do not eat as healthy as they should but there are also many folks who do not eliminate any food groups at all from their diets and are still deficient in multiple vital nutrients due to the food choices they make.
The trouble with veganism is that many people think it is simply living on fruit, vegetables and things like bread, fries and other unhealthy things. To become vegan, one should take the time to slowly make the change as the diet is quite radical, and also to properly research their foods so that they don't become deficient and unhealthy.