Heat oil in a large saucepan or dutch oven over medium heat. Add onion and cook until translucent, but not browned, about 5 minutes. Add garlic and cook 1 minute more.
Add chipotle, ketchup, vinegar, brown sugar, soy sauce and water and bring to a boil then reduce heat to medium-low.
Add pork tenderloin pieces to the sauce and bring to a rapid simmer. Cover the pot and cook, stirring occasionally, until cooked through (71°C/160°F on a meat thermometer, or remove one piece and cut through centre to ensure it's no longer pink inside), about 30 minutes.
Transfer pork to a cutting board. Bring the sauce back up to a low boil and allow it to reduce and thicken slightly.
Using a fork in each hand, pull the pork apart into bite-sized shreds, working with the grain of the meat. Return shredded meat to the sauce, stir to coat.
Notes
1 3/4 lbs pork tenderloins is 2 average tenderloins
Serve in soft buns or tortillas, topped with pickled onions, sour cream, avocado, cilantro (and/or other desired toppings!)
Because pork tenderloin is so tender, you don't need to cook it for a long time to get the tender results you'd get from slow-simmering a tougher cut.