A pointer points to an address. The & (address of) operator gets the address of a variable. This is then treated as a pointer. Since all pointers are the same size, a pointer of one type can be cast to a pointer of another type (as long as due care is taken). So, the pointer to int is cast to (treated as) a pointer to unsigned int (8 bit) (also known as byte) data, which is all that the vw_send function knows how to deal with.
It would be nice to see the complexities hidden in the virtual wire library, but for now, you need to make your data fit the methods provided.