The unseeded, 34-year-old Castano advanced with a hard-fought 2-6, 6-4, 6-4 victory over sixth-seeded Ukrainian Elina Svitolina.
The second-seeded Arruabarrena-Vecino breezed past Argentina's Paula Ormaechea 6-3, 6-1 in a 1-hour, 7-minute affair.
Arruabarrena-Vecino will be seeking her second career WTA Tour title. Her only other victory came in Colombia at last year's event in Bogota.
Swim Week Miami Beach 2014 [PHOTOS]