As you know, I always say that food is more powerful than any drug or supplement. How true this is in the case of vinegar. Scientists are fast discovering that vinegar is, without question, one of the most powerful foods that a guy can take to preserve himself over the years. It recently gained notoriety for its ability to keep the fat off of your midsection. In fact, vinegar helps your body to lose the most dangerous kind of fat: organ or visceral fat. Visceral fat is abdominal fat in your stomach and belly region that is very strongly associated with heart disease.
The study involved found that the acetic acid in vinegar, which is its main component actually, upregulates the genes that burn fat in the liver.  In other words, the acid in vinegar causes your liver to burn more fat! The study involved was on mice and found that the mice fed a vinegar compound had around 10 percent less body fat than the control mice. There were actually two vinegar groups, a low and high dose group and both had much less body fat. In other words, vinegar is one of our truest friends to keep you from gaining weight, a problem for almost everyone as they age or if they live in a Western society.
In some ways this is should be no huge shock as Vinegar has already been found to blunt insulin and glucose responses. This is incredibly important as one of the things that ages our body most quickly is high levels of blood glucose and insulin, especailly as we get older. Vinegar has been shown to greatly help in this regard and helps blunt the spikes. One study of healthy volunteers found that vinegar significantly lower glucose response at 30 and 45 minutes after a meal and insulin responses at 15 and 30 minutes after. 
That same study - and is incredibly important - found that vinegar increased satiety for up to two hours after a meal. In other words, participants found that vinegar decrease their appetite. Vinegars appetite suppression has been discussed in other research as well.  So vinegar not only helps burn off fat but decreases ones appetite while it's at it. Again, this is better than any drug and with no side effects whatsoever!
One question that may cross your mind is if vinegar would have a positive or negative effect on testosterone. The answer is that we do not know at this point. However, there is a good chance that it will help with preserving testosterone after a meal with substantial carbs in it. The reason? Scientists have just recently found out that men who drink a glucose drink experience decreased testosterone for at least two hours afterward. Thus there is a good chance that anything that subdues the glucose spike after a meal with a high glycemic load will help maintain testosterone. Of course, that remains to be proven through studies.
In addition, Metabolic Syndrome and being overweight are huge risk factors for both low testosterone and erectile dysfunction and vinegar can definitely help both of these conditions through strengthening the body's insulin response and helping men lose visceral fat, respectively. In fact, one study showed, at least in diabetics, improved insulin sensitivity after consumption of vinegar. 
The good news does not stop there: one animal study showed that vinegar lowered blood pressure as well, another huge risk factor for erectile dysfunction.  In other words, the odds are extemely high that vinegar can help manage or even partially reverse two of the largest risk factors for your sex life and libido.
CAUTION: Remember that vinegar's primary constituent is acetic acid, which is hard on enamel. It is best to at least rinse your mouth out or brush your teeth after a meal with vinegar in it. Also, it is possible to take too much vinegar - it can harm, i.e. "burn" the esophagus and/or mouth.
1) J Agric Food Chem, May 26 2009, Epublished ahead of print; T Kondo, Kishi M, et al, "Acetic Acid Upregulates the Expression of Genes for Fatty Acid Oxidation Enzymes in Liver to Suppress Body Fat Accumulation".
2) Eur J of Clin Nutr, 2005, 59:983-988
3) Brit J of Nutr, May 2006, 95(5):916-924
4) Nutr Rev, 2000, 58:163-169
5) Diabetes Care, 2004, 27:281-282