Thoroughly wash the cucumbers, bell peppers, and red onion, then pat them completely dry with a clean kitchen towel. Thinly slice the cucumbers into rounds and the bell peppers and onion into even strips. Place all the sliced vegetables into a large bowl or a glass storage container so there’s enough room to mix without crushing them.
Add the cooked chicken breast to the bowl, making sure it is chopped or shredded into bite-sized pieces. Gently toss the chicken with the vegetables using a spoon or clean hands so it’s evenly distributed without breaking the vegetables.
In a small saucepan or heatproof bowl, combine the vinegar, water, salt, black pepper, and optional sweetener. Warm the mixture gently over low heat, stirring occasionally, just until the salt fully dissolves. Do not boil, as overheating can make the pickling liquid too sharp.
Remove the pickling liquid from the heat and allow it to cool slightly for a minute or two. Once warm but not hot, pour it evenly over the chicken and vegetables, ensuring the liquid reaches all areas of the bowl.
Gently toss the salad until all ingredients are well coated and mostly submerged in the pickling liquid. Take care not to overmix, as the vegetables should remain crisp and intact.
Cover the container tightly and place it in the refrigerator for at least 30 minutes. For deeper flavor, allow it to marinate for 2–4 hours, stirring once or twice during chilling to help the flavors distribute evenly.
Serve the salad chilled for the best crunch and refreshing flavor, or divide it into individual airtight containers for easy meal prep throughout the week.