Combine the vinegar, soy sauce, and sugar in a bowl. Mix well. Microwave for 15 seconds. Set aside.
Rub 1/4 teaspoon salt and 1/8 teaspoon ground black pepper on the ahi tuna. Let it stay for 15 minutes.
Heat-up the grill. Grill the tuna for 3 to 5 minutes per side.
Remove the tuna from the grill. Slice into cubes. Set aside.
Heat the oil in a deep cooking pot.
Once the oil is hot, deep fry the tofu (two pieces at a time) until the color turns golden brown.
Remove the tofu from the cooking pot. Let the excess oil drip. Slice the tofu into cubes. Set aside.
Combine the tuna, tofu, onion, and green pepper in a mixing bowl. Toss.
Pour-in the vinegar mixture. Gently toss until all the ingredients are well blended.
Transfer to a serving plate or bowl.
Serve. Share and enjoy!