The execution of the agreement by Vivendi's management board followed "the receipt of positive opinions from the employee representative bodies of the companies of Vivendi, Havas and the Bolloré Group", Havas said in a statement.
A due diligence process has also been completed.
The deal is still subject to the approval of the competition authorities.
Yannick Bolloré, chief exécutive of Havas Group, said : "We look forward to joining forces with the Vivendi group. Vivendi's ambitious strategy for accelerated development towards a global communications leader will enable us to develop even more quickly. Vivendi will provide us with, on one hand, new strategic and financial resources to enrich the expertise we offer to our clients and, on the other, broader opportunities for our talents. We thank the Vivendi teams for their trust and support."