The description a little misleading. Rather than completing every race from the Velocity pack, you have to win every race (finish with gold) with the 5 Velocity speed pack cars, including the races that are not part of the DLC pack.
There are 5 cars in the Terminal Velocity DLC, and each car has 8 events to complete for a total of 40 events. Beat the gold medal score for all these events to unlock this achievement. "Terminal 5" will also be unlocked as soon as you've used each car.
These are the cars:
BMW 1M 2012
Audi RS3 2012
Ford Fiesta ST 2012
Alfa Romeo Mito 2012
Most Wanted Porsche 918 Spyder 2013
The first 4 cars are available as soon as the DLC is purchased. The Porsche 918 Spyder is earned by winning the new 918 Spyder Most Wanted event.