In a small bowl, combine brown sugar with garlic powder, onion powder, paprika, cumin, chilli powder, oregano, salt, and pepper.
Season the pork with the seasoning mixture from all sides.
On the Instant Pot, press on SAUTE-NORMAL setting. When the HOT message is displayed, add the seasoned pork and brown it from all sides in 2 batches (2 minutes per side). Remove the pork onto a plate.
Add chicken stock, and deglaze the pot with a wooden spoon scraping off any bits stuck to the bottom of the pot. Switch off the sauté setting.
Add the vinegar, and stir in the barbecue sauce. Return pork to the Instant Pot.
Secure the lid and set the vent to SEALING. Select the PRESSURE COOK/MANUAL-HIGH setting and set the cooking time for 45 minutes at high pressure. The Instant Pot will take around 10 minutes to come to pressure and start cooking. When the cooking program ends, allow the steam to naturally release for 10 minutes, then quickly release the rest of the steam.
Carefully remove the lid, and transfer the pork onto a cutting board or a baking sheet. Using 2 forks, shred the pork into bite sized pieces. If there are large pieces of fat, discard them.
Simmer the sauce using the SAUTE setting until it thickens (about 15 minutes). Return the shredded pork to the liquid.
Serve immediately with extra bbq sauce if desired. I like to serve on buns with slaw and pickles.