Trim the bottoms and tops of the navel oranges, grapefruits and blood oranges, then cut away the peel and pith with a paring knife. Slice the fruit crosswise and arrange on a rimmed platter.
Combine the lemon juice, sugar, 1/4 cup water and the vanilla seeds in a small saucepan over medium heat. Bring to a simmer, then remove from the heat and let cool to room temperature.
Pour the syrup over the fruit, cover and refrigerate at least 2 hours or overnight.
Photograph by Con Poulos