Viadeo is already established in a number of European countries, with more than a million members on the Continent.
It helps professionals find clients, suppliers, business partners, jobs and employees.
The brief - won after a three-way pitch - will involve consumer and B2B media relations, viral marketing and contact with prominent bloggers.
Brands2Life's remit is to explain the benefits of the networking website to professionals, generate media interest in the UK launch and increase the site's member base.
Brands2Life associate director Gareth Thomas said: ‘Online network sites are no longer just for kids. There is a growing trend of business people using the web as a way to network with other professionals.'