Preheat your oven to 300 degrees. Score the meat's skin with a sharp knife in a cross-hatch pattern. Combine garlic, onion, oregano, cumin, chili, salt and pepper in a food processor, adding oil in a drizzle and scraping down sides as necessary, until mixture turns into a paste. Blend in the vinegar. (If you don't have a food processor, you can do this by hand, but food processors are really great.)
Rub this mixture into the pork, getting it, as Mark Bittman says, "into every nook and cranny." Put the meat in a roasting pan and fill bottom with about 1/2 inch of water. Roast the pork for several hours (a 4-pound shoulder may be done in 3 hours), turning every hour or so and adding more water as necessary, until meat is very tender. Finish roasting with the skin side up until crisp, raising heat at end of cooking if necessary to get the browning you'd like.
Let meat rest for 10 to 15 minutes before cutting it - meat should be so tender that cutting it into uniform slices is almost impossible; rather, whack it up into chunks. Serve with lime! Delicious.