In a large bowl, whisk together the olive oil, soy sauce, balsamic vinegar, and maple syrup until well combined. Add the salt, black pepper, dried oregano, and red pepper flakes if using, stirring to distribute the spices evenly. This creates your flavor foundation—the acidity and sweetness will help caramelize the vegetables on the grill while the oil ensures they don't stick.
Cut the eggplant into 1-inch thick rounds, slice the zucchini into thick rounds, cut the carrots diagonally into 1/2-inch pieces, and cut both bell peppers into wide strips. Add all prepared vegetables to the marinade bowl and toss gently but thoroughly to coat everything evenly. I find that letting vegetables marinate for at least 30 minutes (or up to overnight) allows the flavors to penetrate and helps them release moisture, which actually prevents them from becoming too watery on the grill.
About 30 minutes before grilling, heat your grill to medium-high heat (around 400-425°F). Once preheated, oil the grill grates thoroughly using a folded paper towel dipped in oil—this prevents sticking and helps create those desirable grill marks. Ensure the grates are clean and the grill is hot enough that the vegetables will sear immediately when they hit the surface.
Remove the vegetables from the marinade using tongs or a slotted spoon, allowing excess liquid to drip back into the bowl. Arrange vegetables in a single layer on the hot grill grates—work in batches if needed to avoid overcrowding. Grill for 4-5 minutes per side until you see nice caramelization and grill marks, brushing with the reserved marinade every 2-3 minutes using a basting brush. The total cooking time will be 10-15 minutes depending on vegetable thickness and grill temperature. I like to move the vegetables occasionally to prevent charring in one spot, while still allowing those beautiful caramelized flavors to develop.
Transfer the grilled vegetables to a serving platter and arrange them in an appealing way. Squeeze fresh lemon wedges over the top to brighten the flavors, and garnish with chopped fresh parsley for a pop of color and freshness. Serve warm or at room temperature—these vegetables are delicious either way and actually develop even deeper flavors as they cool.