Add the vanilla extract, sugar, 1 cup water and orange juice to a medium saucepan. Whisk the mixture over medium-high heat until the sugar has dissolved. Add the peaches and plums and bring to a boil. Reduce the heat and simmer until the fruit is tender and the syrup has reduced slightly, about 15 minutes. Let cool for 15 minutes.
In a medium bowl, beat the cream with an electric mixer until thick. Add the confectioners' sugar and vanilla and continue to beat until the cream holds soft peaks. Add the mascarpone cheese and beat until thick and smooth.
Place the shortcakes on serving plates. Spoon some of the poached fruit on the bottom half of each shortcake and top with a dollop of the whipped cream. Add the top of the shortcake and drizzle with the fruit syrup.