For the vinaigrette:
Whisk the vinegar, honey, salt, and pepper together, in a small bowl, until the salt is dissolved. While whisking constantly, pour in the olive oil. Stir in the chives, taste, and adjust seasoning as desired. Set aside until ready to use.
For the salad:
Heat the grill to medium (350 degrees F) and rub the grate with a towel dipped in oil. Brush the vegetables all over with oil and season well with salt and pepper, to taste. Grill the vegetables, turning rarely, over medium heat until soft and charred, about 10 minutes for the onions and 15 minutes for the peppers. Remove the vegetables from the grill to a cutting board and let cool slightly. Cut the onion rings into quarters and cut the peppers into 1-inch cubes. Reserve.
Season the steak generously with salt and freshly ground black pepper. Grill the steak until desired doneness, about 4 minutes on each side for medium-rare. Transfer to a cutting board and let rest for 5 minutes.
Cut the steak against the grain into 1/2-inch slices and add to a medium bowl. Pour 1/3 of the vinaigrette over the steak and toss to coat.
Combine the peppers, onion, watercress or arugula, tomatoes, and the remaining vinaigrette in a large serving bowl, and toss to combine. Top with the steak, adjust seasoning, as desired, and serve.