In a medium glass bowl, whisk the lime juice, vinegar and oil until slightly thickened. Season with salt and pepper. Add the red onions, cilantro, serrano chiles and garlic and toss gently until the vegetables are coated. Add the beans and stir to combine. Season with salt and pepper.
Brush the zucchinis on both sides with olive oil. Sprinkle with salt and pepper. Grill the zucchini on a lightly oiled grill rack until tender, turning over once, about 4 minutes per side. Cut the zucchini crosswise into 1-inch pieces. Add the zucchini to the bowl with the dressing and beans. Toss to combine and serve.