Combine the sugar with 1/2 cup water in a small saucepan. Bring to a boil over medium heat. Stir a few times and allow to simmer until the sauce turns deep brown, about 15 minutes.
Meanwhile, have some extra hot water ready on a back burner. Once the sauce reaches the desired color, carefully add 4 to 5 tbsp hot water to slow the cooking and thin the sauce. (Be sure to hold the pan away from you so that none of the hot caramel splatters on you when you add the water). If necessary, add more hot water. The sauce should only be thick enough to coat the back of a spoon. Set aside to cool.
Pound the shallots in a mortar and pestle or mince by hand. Transfer the shallots to a mixing bowl and combine with the fish sauce, soy sauce, salt, vegetable oil, and cooled caramel marinade. Stir well to blend.
Add the pork slices and let marinate for 20 minutes. Meanwhile, heat a broiler or light a charcoal or gas grill. When the broiler or fire is very hot, cook the pork until just done, about 2 minutes on each side. Let the pork rest for 10 to 15 minutes, and cut into thin strips.