Vitamin D - If you live in the UK, everyone should be taking vitamin D supplements at least during Autumn and winter.
Vitamin B12 - We all know that you can't get B12 naturally from a plant-based diet. It can be added to products like plant-based milk, which is great but make sure it is enough. Nutritional Yeast is also a good form.
Omega 3 - Naturally found in fish, which you won't get on a plant-based diet. It is also in seeds and nuts (milled flaxseed is what I take everyday along with walnuts). If you are not a seed or nut fan, you should look at your levels.
Calcium - It is added to plant-based milks and yoghurt as well as tofu. It is also in green veggies. Make sure you have at least 3 different sources a day.
Zinc - Can be found in wholegrains, beans, nuts and seeds. If your diet is heavy in processed plant-based foods, you might not be getting enough.
Iron - On a plant-based diet, you need vitamin C in your system to absorb Iron. Are you getting enough of both?