The former Espanyol wing-back returns to the LaLiga on-loan to the end of the season.
Aaron moved to Mainz over two years ago and he's managed 60 Bundesliga appearances and also was a regular with Spain U21.
The defender, 23, is already in Vigo and is placed under the orders of Eduardo Coudet after passing a medical examination and signing a contract Thursday. He will wear the number 19.
The loan deal includes a permanent option.