There's several way of using a vfo_t. For most cases, using RIG_VFO_A, RIG_VFO_B, RIG_VFO_CURR, etc., as opaque macros should suffice.
Strictly speaking a VFO is Variable Frequency Oscillator. Here, it is referred as a tunable channel, from the radio operator point of view. The channel can be designated individualy by its real number, or using an alias. Aliases may, or may not be honored by backend, and are defined using high significant bits, like RIG_VFO_MEM, RIG_VFO_MAIN, etc.