Actually it used to mean a certain amount of precious metals, although not anymore. In fact its illegal to carry around more than 100$ worth of gold in preference to banknotes in the United States. Anyways, paper money is commonly accepted as the currency and is obviously backed by powerful people/orginization aka the United States. Caps are the same way. So, it makes sense that New Vegas could have caps as the currency.
The NCR could also be fading in power. They could have had a drawn out war with the BOS since FO2, and could have even lost, or been drained of manpower and resources. Or, infighting could have caused a decline. Anyways, I am sure New Vegas was outside their area of control, so using an NCR currency does not seem likely.
