Not having plenty water or carbs can make you feel sleepy, grumpy and agitated. Not having enough Vitamin K can leave you vulnerable to bruises or cuts. Not having enough B complex or Zinc leaves your body in a lethargic state. Not having enough Vitamin C and water makes your body unable to detoxify well. Not having enough nitrates keeps you from having explosive power to lift or do heavy short burst physical work.
I am an omnivore from India, so I have my eggs, peanut butter cocoa milk, do MMA and workouts, am my own nutritionist and trainer and have my own gym at home. This is all hobby stuff for me.
I cannot really guide you about vegan diets though, as I do not find it a healthy fad.