RSM won the business after a two-way pitch and replaces ProAction, the incumbent agency for the past two seasons.
Vauxhall - which this year celebrates its centenary - is aiming to become the first manufacturer in the history of the British Touring Car Championship to land manufacturer's, team's and drivers' titles for three successive years.
Director Juliet Jarvis heads a three-strong team at RSM - which stands for Result Sports Management - reporting to chief press officer Robin Bartlett.
Bartlett said the work had been repitched because Cambridgeshire-based ProAction had gone out of business in November last year.
RSM will promote Vauxhall's participation in the BTCC, handling both on and off-track team and driver PR work, during the season, which runs from April to September.
The Williams team, working in co-ordination with Vauxhall's 15-strong in-house department, will target the motorsport, automotive, regional and consumer media.
Last autumn Vauxhall hired Bell Pottinger on a corporate and consumer PR remit (PRWeek, 11 October 2002). It is not affected by the RSM hire.