So you've accounted for protein, but you need a lot more than that. For example, without vitamin C, you'll get scurvy. Without B12, your nervous system can be permanently damaged.
I'd highly recommend a diet of more than just beans, rice, potatoes, and fish oil. You need to include some fruit and vegetables, and if you're going to be totally vegan, some sort of B12 source (fortified foods or a vitamin pill).
Sure, that's why I suggested a multi. If you want to get the vitamins naturally I'd suggest some cabbage and the occasional egg to get your C and B12. Actually, eggs and cabbage will pretty much cover all your nutritional needs but aren't the cheapest way to bulk macronutrients. Still, 50 cents worth of cabbage and eggs a day would be enough to cover your vitamin and mineral needs.
I wouldn't eat that diet, personally. I'm a paleo guy and value quality veggies and meat. I'm on a student budget and I invest most of it in food. If I were raising a family of 6 on my current wage though, I think rice/beans/eggs/cabbage/fish oil would be a pretty solid, reasonable diet on the cheap, throwing in as much extra meat and veggies as I could afford.
I'd highly recommend a diet of more than just beans, rice, potatoes, and fish oil. You need to include some fruit and vegetables, and if you're going to be totally vegan, some sort of B12 source (fortified foods or a vitamin pill).