Everyone seems to have different opinions on this , so here are mine but don't shout if i miss something out.
B12 must be got from several portions of fortified foods a day or supplementation
Iodine is only found in a few vegan foods i.e. seaweed or fortified foods so if you don't eat these i'd supplement.
Calcium is a bit of a sketchy area as far as supplementation goes as a low fat diet helps bioavailability and the lower protein and higher alkalinity of vegan diets improves conservation of calcium. Also the body adapts to lower calcium intakes by increasing the rate of absorption, provided Vit D is sufficient. However as previously stated high oxalate or phytate in the diet from foods such as spinach or swiss chard impairs bioavailablity.
Vitamin D supplementation is generally not necessary if you get 5-15 minutes of sunlight exposure a day during spring, summer and autumn(fall). This provides adequate vit D over winter months. However, if you do not, or are dark skinned/Asian you may benefit from supplementation as Asian people have been found to have a blunted response to exposure to sunlight.
Selenium - research on this nutrient varies, some studies show vegan diets to be deficient, others not. Brazil nuts, other nuts, seeds, soya beans, mushrooms, grains and bananas are good sources so if you eat these you should be fine, if not supplement.
Zinc intake is usually similar to or higher than omnivores but is also inhibited by phytates. vegan sources include wholegrains, lentils, seeds, almonds and oats.
Iron- Pulses, wholegrain breads and cereals or fortified cereals are good vegan sources of iron. If eaten daily and with foods or drinks high in Vit C(glass of Orange juice, mango etc) and without foods/drinks containing tannins, oxalates and phytates(tea, spinach etc) which inhibit Iron absorption, iron levels should be OK.
Omega 3 fatty acids- linseed/flax seeds, hemp seed or walnuts or oil, sweet potatoes, soya beans, pumpkin seeds are good sources and help to balance omega 6's to omega 3's. Also using olive, rapeseed or soya oil rather than sunflower, safflower or corn oil helps.
Hope this helps a little