The victory follows a lengthy pitch against the incumbent, which was sold by Lowe & Partners Worldwide earlier this year. A number of smaller agencies are also understood to have pitched.
DLKW, which declined to comment on the story, will produce above-the-line advertising for each of Vauxhall's 450 dealerships.
The win continues the migration of the £63 million Vauxhall business to DLKW. A string of wins over the past few years means DLKW now works on the Corsa, Meriva, Astra and Agila brands.
It most recently picked up the £12 million brief to relaunch the Astra and the national television and press campaign broke last week.
In 2002, the agency also scooped the £15 million Vauxhall retail account, also from Lowe Broadway, to promote the UK dealer network.
In February, DLKW experienced its first loss when the main Lowe agency won the launch brief for the Tigra sports car. Lowe also handles the Vectra, Zafira, Signum and Vauxhall Vans accounts.
Lowe Broadway has now lost a significant slice of its business. It also works for HSBC, which is reviewing all its advertising services.
Vauxhall is the second-biggest-selling car manufacturer in the UK after Ford and has the country's second-largest dealership network. Its new marketing director, Andy Gilson, is tasked with fulfilling Vauxhall's long-term aim of knocking the market leader off its top spot.
Last year, Vauxhall's sales increased by 2.45 per cent and its market share grew from 12.4 per cent to 12.6 per cent.