Velez captain Fabian Cubero gave away a penalty and was sent off in the 27th minute but Sosa saved Ignacio Scocco's penalty to deny Newell's an equaliser after Pratto's early goal in Saturday's match on neutral ground in Mendoza.
The title, Velez's second of the season, has however produced the anomally of two teams that can claim to be Argentine champions at the same time.
Velez qualified for the grand final as winners of the "Inicial" championship in December but had a poor "Final", won by Newell's this month, in which they finished 14th with 20 points from 19 matches.
Newell's were the best team overall having come second in the Inicial (former Apertura) before winning the Final (ex Clausura) and the Argentine FA said Saturday's result did not invalidate the titles already obtained in the season.
For the past two decades, Argentina has had two champions per season, only introducing a grand final this year, which prompted Newell's coach Gerardo Martino to say he had never before taken part in a lopsided title match.
"Neither as trainer nor player have I taken part in a deciding match in which the prize for one team was greater than that for the other," Martino told reporters.
Velez won a cash prize worth about $335,000, a place in the 2014 Libertadores Cup and in the Copa Sudamericana, equivalent to the Europa League, later this year.
Newell's place in the Libertadores, South America's top club competition, next year was already in the bag as "Final" winners.
Martino said the most pleasing aspect of the match for him was his team's performance ahead of their 2013 Libertadores semi-final first leg against Brazil's Altetico Mineiro at home on Thursday.
"I saw again the Newell's we all love," Martino, whose side had played poorly in their last league match last weekend with the title already secured, told reporters.
"The impression is good so I'm quite calm. I liked the look of the team although we're left with the taste of defeat."
Victory gave Velez, Libertadores Cup winners in 1994, their 10th league title and fourth since 2009 under coach Ricardo Gareca. Newell's have six.