1. Liberally season tenderloins with salt. Coarsely grind star anise, coriander, and fennel using a mortar and pestle or a spice grinder. Add five-spice powder, black pepper, ginger, cinnamon, red pepper flakes, and salt; pound or whirl to combine. Rub spice mixture on pork until thickly coated on all sides (save remaining mixture for another use). Let pork sit at least 1 hour (at cool room temperature, covered) or up to 1 day (covered and chilled).
2. Heat 1 tbsp. olive oil in a 10- to 12-in. heavy frying pan over medium-high heat. Add 2 tenderloins and brown well on all sides, 10 to 15 minutes, drizzling in additional olive oil as needed. Meat is done when it registers 140° on a meat thermometer (cut to test--it should be cooked but still rosy). If meat is not done, cover pan, lower heat to medium, and cook until thermometer registers 140° (cut to test), up to 10 minutes more.
3. Remove tenderloins from pan, cover with foil, and let rest at least 10 minutes. Cook remaining tenderloin the same way, but brown it over medium heat and check temperature after 10 minutes. Slice meat into 3/4-in. rounds and drizzle with vinegar.
Note: Nutritional analysis is per serving.