Preheat oven to 400°. Place baguette slices on a rimmed baking sheet and toast in oven until golden brown, 8 to 10 minutes. Let cool.
Increase oven heat to 425°. In a large bowl, toss grapes, vinegar, olive oil, and salt to coat. Spread on a parchment-lined rimmed baking sheet and roast until grapes are caramelizing and beginning to pop, 15 to 20 minutes.
Combine ricotta and lemon zest in a small bowl, then spread on toasts. Set a few grapes with their liquid on each toast, then sprinkle with pistachios.