Drain and rinse black eyed peas if using canned. If using fresh, cook until tender and let cool.
Chop the cucumber, cherry tomatoes, red onion, and green beans (if using). Blanch green beans and chill in ice water if needed.
Whisk together olive oil, red wine vinegar, lemon juice, salt, and pepper to make the dressing.
Combine black eyed peas, chopped vegetables, and feta in a large mixing bowl. Pour dressing over and mix gently to combine.
Chill the salad for at least 30 minutes before serving to allow the flavors to meld.