Whisk 3 tablespoons white wine vinegar, 1 tablespoon water, 1 teaspoon honey and 1/2 teaspoon kosher salt in a large bowl. Add 1/2 thinly sliced red onion and toss; let sit, stirring occasionally, 20 minutes. Brush 4 ears corn with vegetable oil and grill over medium-high heat until charred, 8 minutes. Let cool, then cut off the kernels. Add to the onion along with 2 chopped tomatoes, 1 diced avocado and 2 tablespoons chopped parsley; toss. Season with salt and pepper.