A Viceroy (Vicereine if female) is appointed by a monarch to exercise some of the monarch's powers.
Viceroys can exercise these delegated powers within their own lines. A Grand Viceroy or Grand Vicereine -- one per kingdom -- can exercise these powers across the entire kingdom. The time limits on certain powers of monarchs, such as land tracts, cannot be evaded by the use of viceroys; the main benefit of a viceroy appears to be coverage for a kingdom during a monarch's time out-of-world.
The minimum requirements for a viceregal appointment are steep. The candidate must hold enough acres to ascend to a throne in their own right (1000 personal, 20K total). Also, the candidate must complete the entire Path to Royalty except for the final coronation step. Finally, the monarch must grant royal favor and make the appointment. In the case of a Grand Viceroy/Vicereine this is done twice, once for appointment and a second time for promotion.
To exercise their powers the GV/V must remain direct to their monarch and retain 20K acres minimum. A Viceroy/Vicereine no longer direct to the monarch retains the title but will not receive any of the specific offers allowing use of viceregal powers.