Vitamins, veggies. You can always take a vitamin with a high dose of calcium and vitamin d. More than likely though a vegan diet may be a little low in some other nutrients. I suggest seeing a nutritionist for planning purposes. Many vegetables, like broccoli, have high levels of calcium. Others have iron or allow higher absorption of iron (tomatoes). A nutritionist can give you a good healthy plan.
Don't do this. The long term effect of a low fat, vegan diet is malnutrition. Used for a short time (4-6 weeks) a low fat diet can be employed for weight loss. However, the long lerm effects of low fat diets are miserable. A low fat - high carb diet will lead to high Insulin levels, high triglcyerides, low hdl, metabolic syndrome and finally high blood pressure, diabetes and heart disease.