People on a WFPB diet need to insure an adequate intake of vitamin B-12. Vitamin B-12 is made by bugs in the environment. Modern sanitation protects us from many bad bugs, but also the bug that makes viamin B-12. If we consume neither non sanitary water and veggies nor animals that consume non sanitary water and veggies, then supplementation is the way to go.
I use iodized salt. Getting the recommended amount of salt as iodized table salt (not salty prepared foods) will take care of iodine need on a WFPB diet. Health-food stores should have dulce (a form of seaweed) or drops for iodine.
Other than for B-12 and iodine, be wary of supplementation. For example, people that get beta-carotene in their food are healthier than people that don't. But when beta-carotene supplementation is compared to a placebo, the people on the placebo are healthier. One possible reason is that beta-carotene is not alone. There is alpha-carotene all the way to omega-carotene and beyond — about 50 usable carotenoids in all. So the body's attempt to use a combination of these is defeated if all the pertinent receptors are filled with beta-carotene.
The stories of failed attempts to improve health by supplementing with vitamin E or lycopene are the same as the story with beta-carotene.