How does vinegar kill bacteria in meat?

Yes, vinegar DOES kill many bacteria and viruses, including many very dangerous ones that can be in food such as E. Coli O157:H7 (see for example scholarly research that shows this...

Freezing can damage bacterial cells, but it won't kill them all off. The bacteria can recover and start to grow again once the meat is thawed.

Yes. If the meat has been reheated to 165°F, vegetative bacteria should have been killed off. Bacterial toxins are a different matter. Most toxins are not destroyed by heating.

yes, it depends on the species of the bacteria. Some bacteria can produce toxin to inhibit the growth or kill the other bacteria if the bacteria doesn't have the specific anti-toxin.

Generally yes, the acidic content in the vinegar will kill bacteria.