No, you can't write a skin in java. You don't need to know vb to write/make a skin. A skin is a series of images. Each screen contains 2 - 4 images that rr uses to display the screen. On a typical screen you would have 4 images, each image is a different layer and depending on what you do, RR copies an area from one layer to the top layer to give the effect of the action that either you or RR is doing. Some examples are when you put your mouse over an area that is defined as a button, RR copies that button area from the "mouse over" layer to the top which makes the button highlighted, same goes for any other action like when you click a button area. The only other way to create a skin is to write it in flash if you know it.
Hope that helps you to understand how rr is skinned. You might want to look in the skin subforum and check out the FAQ, as there is a better description on how to go about making your own skin to fit your needs.