While no decision has been made, Vivendi is looking at boosting its holding to a range of 10 percent to 15 percent as early as next month, the person said, asking not to be identified because the deliberations are private. Vivendi, the French media company chaired by billionaire Vincent Bollore, is set to become Telecom Italia's largest shareholder, with a stake of about 8 percent that it's taking on from Telefonica SA.