Preheat the oven to 350 degrees . Line the bottom of a buttered 2-by-9-inch round cake pan with wax paper, then butter the paper. Sift the flour, baking powder, baking soda and salt. Using an electric mixer, beat 1 cup sugar and the butter at medium speed until fluffy, then beat in the lemon peel and vanilla. Beat in the eggs 1 at a time, then mix in the buttermilk on low speed. Working in 3 batches, mix in the flour mixture.
Pour the batter into the prepared cake pan and bake until golden and a toothpick comes out clean, 35 to 40 minutes. Let cool on a rack for 10 minutes, then run a sharp knife around the edge. Invert onto the rack and let cool completely. Transfer the cake to a cake stand.
Meanwhile, in a large saucepan, bring the remaining 1 1/2 cups sugar and 1 cup water to a boil, stirring until the sugar is dissolved. Simmer for 5 minutes. Stir in the citrus slices and simmer for 10 minutes. Drain and cool the fruit in a sieve set over a bowl; return the syrup to the pan and boil gently until reduced to 1 cup, about 10 minutes.
Poke the cake all over with a skewer, then pour 3/4 cup of the syrup slowly and evenly over the cake. Arrange the citrus slices decoratively on top. Drizzle the remaining syrup over the fruit.