To prepare marmalade, warm olive oil and butter in a saucepan over medium heat. Add plum, shallots, tomatoes, and garlic; bring to a simmer, and cook 30 minutes, stirring often. Stir in sugar, harissa, thyme, and vanilla bean. Simmer 20 minutes or until thick. Remove from heat; cover and let stand 30 minutes. Discard thyme sprig and vanilla bean; stir in basil and 1/4 teaspoon salt.