google vegan athletes. there's your proof it's healthy. There are olympians, bodybuilders, cyclists....
If they ask you where you get your [protein/calcium/iron/B12] ask them where they get theirs. The only thing they know is that they get calcium from dairy, and in reality most omnis don't get enough calcium daily anyway.
If you look at statistics, vegans have higher levels of essential nutrients per capita than the general population because we pay attention to our diet. You'll get your protein from lentils/legumes/nuts/seeds/grains/soy products.
You'll get your calcium from fortified soy milk/vegetables (yeah, its not ONLY found in dairy.)
You'll get your iron from dark greens/nuts/soy/grains, you'll make sure your absorption is better by combining these foods with high vitamin c foods (a little oj is all you need or a some fresh tomato...) You'll make sure you don't inhibit your iron uptake by not drinking tea/coffee or red wine with iron rich meals.
You'll get B12 from a supplement, nutritional yeast or a multivitamin.
Then ask them where they get their iodine, zinc, potassium, magnesium, and essential amino acids (omegas).
They won't even know they need them.
You'll get yours from various fruits and veges and flaxseed and canola oil. (look up specifics if you like, strawberries for example are very high in iodine.) You can also buy omega and iron fortified soy milk.
You'll get asked every question above and more, and not just once.
A website you'll find if you google 'vegan health' has excellent fact sheets on nutrition. print them. hand them out.