| | you need to supplement a vegetarian diet
You need to supplement a vegan diet with B12, zinc, maybe calcium, perhaps iron and possibly some other things like sulfur.
You need protein so being aware of how seeds and nuts compliment grains, for example, is helpful to be mindful of when eating or planning meals. Animal protein is complete protein but only a few vegan sources are complete protein. So making the right combos of incomplete proteins will make your choices work better for you.