Step 1) Combine fish sauce, honey, minced garlic, minced onions, Five Spice and oil in a resealable plastic bag. Make several slits in pork tenderloin with a sharp knife; place meat in bag and marinate a minimum of 30 minutes in the refrigerator.
Step 2) Preheat grill to medium-high heat. Remove pork tenderloin; discard remaining marinade. Grill tenderloin 15 to 20 minutes, turning frequently, or until a minimum internal temperature of 155°F is reached.
Step 3) Remove from grill; tent with foil and let sit 5 to 7 minutes. Serve immediately or cool before slicing for Banh Mi sandwiches.