Preheat the oven to 350°. Butter and flour a 9-inch springform pan. In a small bowl, whisk the flour with the salt and baking powder. In a large bowl, whisk the eggs with the granulated sugar until pale yellow. Whisk in the milk, olive oil, melted butter, lemon zest, orange zest and vanilla. Fold in the flour mixture, followed by 1 1/2 cups of the grapes. Scrape the batter into the prepared pan.
Bake the cake for 15 minutes. Dot the top of the cake with the remaining 1/2 cup of grapes and bake for about 40 minutes longer, until a cake tester inserted in the center comes out clean. Transfer the cake to a rack, carefully remove the ring and let cool to room temperature.
Meanwhile, in a large saucepan, boil the grape juice over high heat until reduced to 1 cup, about 25 minutes. Let the grape syrup cool to room temperature.
Remove the cake from the base and transfer to a serving platter. Dust the top with confectioners' sugar. Cut the cake into wedges and serve, passing the grape syrup at the table.
The grape syrup can be refrigerated for up to 2 days. Serve the syrup chilled or at room temperature.