I don't know about anyone selling Vonage in the DR, but a lot of places sell Magic Jack. Works very well from the DR to US and vice versa. Check with computer stores on this. In the US the new Magic Jack is $60, here about $100.
If you buy one there are two models. The Original and the Magic Jack Plus. The only difference is that with the plus you only need a computer to set it up, and then you can plug it into your router without a computer and it will work. The Original is less money, but works the same. Only it needs to be plugged into a working computer.