Soak at least 10 wooden skewers in water for at least 20 minutes. The total number of skewers will vary depending on the size of your vegetables and how big you chop them.
Whisk all vinaigrette ingredients together in a small both and set aside.
For the red onion, cut the ends off, peel, then slice in half length-wise. Chop each half into fourths.
Seed and chop each bell pepper into about 1-inch pieces.
Chop the zucchini and summer squash into rounds. All of your veggie pieces should be approximately the same size.
Skewer the vegetables by alternating between red onion, zucchini, and the different colored bell pepper. Lightly coat skewers with cooking spray.
Grill the skewers over medium-high heat for approximately 5-8 minutes per side, or until vegetables are just softening and browning around the edges. Continually brush with vinaigrette while cooking.
Remove skewers from heat and drizzle with a little more vinaigrette before serving.