Place the roast on its side and split in half lengthwise with a sharp knife. Make shallow slashes in a diamond pattern on both sides of each piece. Transfer halves to a flat dish.

Whisk rice vinegar, garlic, fish sauce, soy sauce, sriracha, salt, pepper, and Chinese 5-spice powder together in a bowl. Pour sauce over the meat; turn each piece over several times to thoroughly coat. Let meat marinate for about 30 minutes at room temperature. (Or transfer to a zip top bag and refrigerate for 3 or 4 hours.)

Heat a heavy cast iron skillet over medium-high heat; add 1 tablespoon vegetable oil. Brown meat on each side, 4 to 5 minutes per side. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C).

Transfer to a plate to rest at least 5 minutes before slicing.