As stated, the details of your dish will make a difference. If there are elements in your dish which have absorbed the vinegar there may not be a great deal you can do. If, however, the dish is more of a stew or soup where things are sitting in the liquid, you might try straining off the existing stock/vinegar liquor and replacing with fresh.
Given that you would loose a lot of the complexity and intensity of flavour by straining off the existing stock I would try getting a fresh pan and cook up some fresh onions with plenty of butter, olive oil or even bacon fat if you have that available, plus some more garlic. Take time to really caramelize the onions to get a good depth of flavour and build up a fond in the pan, then deglaze with the new stock and add that mixture to the original dish.
Dilution by adding more of the original ingredients if you still have any will also help.
Check how is tastes when you have combined all of that and see if you have anything else in your cupboards that can add depth of flavour and umami to counteract any lingering vinegariness or 'thin-ness' of flavour from dumping the original liquid. From my own cupboard I'd probably be adding some dried cep powder or a slug of Colatura di alici (an Italian fish sauce made from anchovies) or even a spoon of Marmite or Bovril. You might not end up with exactly the original dish you were aiming for, but if you trust your tastebuds you should still end up with something good.