What food should only be "fortified" in a vegan diet?

Nothing SHOULD only be fortified. All nutrients are available in vegan foods, in the case where intakes of limited nutrients such as vitamin D, B12, DHA, and some minerals such as Iodine are inadequate then fortification helps boost the intake. But that doesn't mean they should only be consumed this way just because they can, if you can get it from natural sources in the diet then that's good.