Combine vinegar and shallot in a bowl; set aside to let shallot soften.
Prepare an ice-water bath. Bring a pot of water to a boil. Add 1 teaspoon salt and green beans, and cook until bright green, about 2 minutes. Using a slotted spoon, transfer green beans to ice-water bath. Add zucchini to boiling water, and cook until just tender, about 3 minutes. Transfer zucchini to ice-water bath. Let cool, then drain.
Toss green beans, zucchini, tomatoes, and cucumber in a large bowl.
Add mustards, herbs, 1 teaspoon salt, and pepper to shallot mixture. Add 1/4 cup oil in a slow, steady stream, whisking until emulsified. Pour vinaigrette over vegetables, and toss. Let stand at least 1 hour to allow the flavors to meld. (Vegetables can be covered and refrigerated up to 2 days; bring to room temperature before serving.)
Preheat oven to 350 degrees. Place bread on a rimmed baking sheet. Drizzle with remaining tablespoon oil, and toss. Toast, stirring occasionally, until golden brown, about 20 minutes. Add croutons to salad, and toss. Let stand 1 hour before serving so that croutons can soften. If you prefer your croutons crisp, add them to the salad just before serving.