Pre-heat your oven to 275F.
Combine the cider vinegar, garlic, Mexican oregano, cumin, chili powder, chipotles and adobo, salt and enough water to make a smooth paste in a blender. Blend until smooth.
Brown the beef in the oil using an oven proof dutch oven large enough to hold the beef. You want the meat to brown up but not crazy brown. If you push it too far the edge of the beef will not get to the meltingly tender stage. This step is critical to getting the big beefy flavour so don't skip it.
Remove the beef from the pot.
Adjust your heat to medium low. Pour the chili/vinegar paste into the pan. It will bump so be ready. Don't wear nice clothes for this step.
Cook the chili/vinegar paste for about 2 minutes, scraping up any browned bits on the bottom of the pot.
Add the chicken stock and return the beef to the pot.
Cover and place in the oven. Cook (braise) the beef for about 4 hours. You are going for fork tender. Start checking around the 3 hour mark. It's like BBQ brisket. It's ready when it's ready. Not before so don't pull it because you are running out of time. If it's taking too long nudge the temperature up to 325F and check every 15-20 minutes.
Remove the beef from the pot. Let the beef cool until you can shred it. Two forks works for this. Your hands work pretty well too.
Pour the liquid into a bowl. Skim the fat from the surface. If you have a fat separator kicking around from Thanksgiving use that.
Pour the defatted braising liquid (sauce) into the shredded beef and mix. Return the meat and liquid to the pot. Bring to a simmer and cook for around 5 minutes. This gets the sauce and the meat to really come together. Taste and adjust salt if required.
Serve and prepare to be told you make better than taqueria beef barbacoa!