In a small mixing bowl combine the balsamic vinegar, olive oil, mustard, fresh rosemary, fresh thyme, and garlic. Pour 3/4 of the marinade into an air lock plastic bag with the pork tenderloin. Place the other marinade in an air tight container in the fridge. Close the top, making sure to release all the excess air. Make sure all of the pork tenderloin is covered in the marinade and let sit in the fridge for about 8 hours (you can marinade for as little as 1 hour, but 8 hours achieves the best flavor).
After the pork is done marinading remove it from the fridge and allow to come to room temperature. While the pork is warming, heat a grill to high heat.
Place the pork on the grill at high heat to sear the edges. Once the pork is on the grill immediately reduce the heat to medium or medium/low depending on your grill. Brush the pork with some of the reserved marinade. Allow the pork to cook for 10 minutes. Brush the top with more of the marinade and flip the pork over and cook for another 10-15 minutes, until the pork reaches an internal temperature of 165 degrees F.