Combine the chicken, cucumber, bell pepper, tomato, onion, beans, and feta in a large salad bowl. In a separate bowl, combine the vinegar and oregano with a few generous pinches of salt and pepper. Slowly drizzle in the olive oil, whisking to combine. Toss the dressing with the salad. You can serve now, but it's best to let this one sit in the fridge for 30 minutes or so, which gives all the ingredients a chance to get friendly.