There is no one "perfect food". In terms of fulfilling human micro-nutrient needs, the closest thing is human blood, but that's damn expensive, and contains cholesterol other bad stuff.
Sweet potato is just 6% protein. It has tons of Vitamin A and other micro-nutrients, like most plants do, but to get the FDA daily dose of protein (and I like to get 2-3 times that) and calcium you'd need to eat 3.3 kilograms of them - so much "carb junk" will make you fat, and you'll be sick of them and craving meat very soon. Peanuts are 18% calories from protein, but 71% calories from fat. You get the idea.
The only way to have a healthy vegan diet is to focus on low-fat foods that are over 20% calories from protein (beans, peas, legumes, select vegetables, best kinds of bread, etc), and "JUST SAY NO" to foods that are under 10% calories from protein (most fruits, sweets, oils, rice, potatoes, etc). So you only worry about macro-nutrients (i.e. protein / cabrs / fat), while micro-nutrients are pretty much taken care of by default from the veggies and beans, plus there's nothing easier and cheaper than vitamin supplements.