Whisk balsamic vinegar, honey, and brown sugar in a bowl until the sugar mostly dissolves. Add olive oil, Dijon, garlic, salt, red pepper flakes, and rosemary; whisk until smooth.
Scoop out 1/3 cup of the marinade, cover, and refrigerate for glazing. Place tenderloins in a shallow dish or zipper bag, pour the remaining marinade over, and turn to coat. Marinate in the refrigerator for at least 2 hours and up to 18 hours, turning once if possible.
Remove pork from the marinade and pat very dry; discard used marinade. For even cooking, tuck the thin tail end under and tie with kitchen twine in 2–3 spots. Let the pork stand at room temperature for 20 minutes while you heat the oven.
Heat the oven to 425°F. Line a rimmed sheet pan with foil and set a lightly oiled rack on top. Place tenderloins on the rack and brush lightly with a tablespoon of the reserved marinade.
Roast for 10 minutes, flip, and brush again with a little reserved marinade. Continue roasting 6–10 minutes more, until the center reads 140–145°F on an instant-read thermometer.
Switch the oven to broil (high) and broil the pork for 60–90 seconds to caramelize the exterior. Watch closely to prevent burning, then transfer to a board and rest for 10 minutes.
While the pork rests, microwave the remaining reserved marinade in a small bowl for 60–90 seconds, just until bubbling and slightly thickened (or simmer 2–3 minutes in a saucepan). Slice pork and drizzle with the warm glaze.