Velez clinched their ninth league title with a match to spare, taking 38 points from 18 matches for an unassailable five-point lead over second-placed Lanus, who lost 1-0 at River Plate.
It was coach Ricardo Gareca's third championship with Velez in three and a half years.
Ferreya scored in the 48th and 84th minutes at the Fortin against bottom team Union, who have gone 22 league games without a victory dating back to last season.
"Chucky" Ferreyra put Velez in front with a header and took his championship tally to 11 with a brilliant second goal, curling the ball left-footed inside the far post from striker partner Lucas Pratto's low right cross.
"I'd like to thank Gareca for the faith he showed in me," the 21-year-old Ferreyra told reporters.
Right back and captain Fabian Cubero, a veteran of all three championship victories under the same coach, added: "[Gareca] knows how much we love him."
River, watched from the heaving Monumental stands by new coach Ramon Diaz, who takes charge on Monday, scored in the 76th minute when Uruguayan striker Rodrigo Mora headed home midfielder Leonardo Ponzio's free-kick from the right.
Diaz, who steered River to seven league and regional titles between 1996 and 2002, came out onto the pitch before kick-off to a standing ovation from the crowd, welcoming him back after 10 years.
"It was a very good farewell for [departed coach Matias Almeyda] and a welcome to his successor," Ponzio told reporters. Almeyda quit during the week and reserve team coach Gustavo Zapata took charge for Sunday's match.
Despite the win and an encouraging second half performance, Diaz will have seen he faces plenty of work to mould another title winning River side.
Lanus lost momentum, dropping five points in their last two matches after a run of seven victories, while Belgrano beat Tigre 1-0 at home in Cordoba to climb to equal second on 33 points.
Boca Juniors, Apertura champions this time last year, could have also gone joint second but lost 1-0 away to outgoing title-holders Arsenal, who won for the fourth time in succession.
Coach Juan Antonio Pizzi's San Lorenzo notched a fourth win in five matches, coming from a goal down to beat Independiente 2-1 at home to keep the Red Devils mired in relegation trouble.
The season is divided into two championships, the Inicial and the Final, which kicks off in February after the summer recess, with the two winners meeting in a grand final in June unless the same team win both.
In previous seasons there were also two championships, Apertura and Clausura, but no overall winner.
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