Bring the water, vinegar and sugar to a boil. Place the onions in a heat-resistant glass bowl. Pour the vinegar over the onion and let rest for 30 minutes. Remove the onion and let cool on the counter.
Grill the bologna on high heat for 1 - 2 minutes per side, or until hot.
Dice the avocado and bologna.
Warm tortillas on the grill or microwave.
Add diced bologna to the tacos and follow with the onions and avocado. Add salt to taste.
Sprinkle cheese and cilantro on top.