>> Tipking says: It is not always advisable to look too deeply into everything in life or we may be starting to wonder why eggs are oval or grass is green. There are reasons for both of these bud do we need to dwell on it...
I have to disagree here Tipking. Had we not looked into why grass is green, we would never have discovered chlorophyl, a pigment vital to photosynthesis, which allows plants to obtain energy from light and the basis to our creation of solar cells. Had we never questioned why an egg is oval, we may never have learned so much about the structural integrity of curved vs flat, and likely never traveled into space or even beyond rudimentary flight.
Having said that, yes... pounding on meat breaks down the toughness of meat by forcing cells to separate (tearing), which is also what mastication does. This also means the meat will be more prone to browning and freezer burn, so only physically tenderize just prior to cooking the meat (and after marinating). If you wish to marinade prior to physically tenderizing, and yet still want the marinade to seep into the meat, use a fork sparingly instead of a hammer liberally.
Vinegar, being an acid, chemically dissolves the membranes surrounding cells, and if left to soak long enough, given sufficient acid, would dissolve the cells themselves (acid neutralizes at a certain point, so it is best not to put too much vinegar in your marinating, should you forget to remove your meat from the marinade. Also, realize not all vinegars have the same acidity, so when trying to determine just what is "the right amount," stick to the same brand/source.
Personally, when I use vinegar to tenderize, i prefer apple cider vinegar (red wine vinegar flavors the meat, but leaves it bitter, while apple cider vinegar also flavors the meat, but leaves it sweet). I then rinse the meat and leave it to soak in a marinade mix for 1-24 hours (depending upon the degree of flavor override I want).
Cooking with vinegar (again, i prefer apple cider vinegar) also works, and at a faster rate than merely storing it, but it will leave you with a slight aftertaste that I don't much care for. I recommend, if you're going to attempt tenderizing while cooking, use either a sweet red wine or apple cider (both of which are mildly acidic, and would thus only marginally tenderize... but they add a nice flavor).