Preheat the oven to 425 degrees F (220 degrees C). Line a large baking sheet with foil. Spread Brussels sprouts, squash, bell pepper, and red onion on prepared baking sheet. Drizzle with 2 tablespoons oil. Sprinkle with salt and pepper; toss to coat.

Roast vegetables in the preheated oven, stirring halfway through, until browned and tender, about 20 minutes. Let cool 10 minutes.

Meanwhile, for vinaigrette, whisk together vinegar, herbs, garlic, and mustard in a large bowl. Slowly pour in remaining 2 tablespoons oil, whisking until oil is incorporated and dressing is smooth and creamy.

Add vegetables to bowl with vinaigrette; toss. Let cool to room temperature (or chill, covered, up to 1 week). Sprinkle salad with pepitas just before serving.