The Havas stake is expected to fetch $384m (£235.3m) and will be sold in part to institutional investors with the remainder going to Havas itself.
The cash from the sale of the French advertising group will be added to the proceeds of the sale of Exposium, Vivendi's trade fair and professional business in France and its local free newspaper group Bonjour.
Shares in Havas climbed slightly this morning €15.68 (£9.61) in Paris while Vivendi's share price dropped 1% to €74.90 (£45.90).
Havas is the world's fourth-largest advertising business -- it owns the Euro RSCG Worldwide network, Media Planning and Snyder Communications.
The line-up of potential buyers for Vivendi's professional and free magazine business is expected include Reed Elsevier, Pearson and Wolters Kluwer.