Preheat the oven to 400 degrees . Line 2 large baking sheets with parchment paper. Fill a medium saucepan with enough water to reach a depth of 1 inch; bring to a simmer over medium heat.
Using an electric mixer, combine the egg whites, sugar and cream of tartar. Place the mixing bowl over the simmering water and whisk constantly until the sugar is dissolved and the whites are warm to the touch, about 3 minutes.
Return the bowl of egg whites to the mixer and beat, first on low speed and gradually increasing to high, until stiff, glossy peaks form, about 5 minutes. Whisk in the vanilla for about 1 minute. Fold in 1 cup pistachios.
Drop rounded tablespoons of the meringue about 1 inch apart onto the prepared pans. Sprinkle some pistachios onto each meringue. Lower the oven to 200 degrees and bake until the meringues are crisp on the outside, about 2 1/2 hours (decrease the oven temperature to 175 degrees if the meringues start to brown).