Bollore spent more than 1.5 billion euros (1.3 billion pounds) on Vivendi shares between February and April in 19 separate transactions, according to Reuters calculations, increasing his stake from 20.5 percent previously.
Because of French rules that grant double voting rights to established shareholders, Bollore's stake gives him effective control at the group's shareholder meetings.
The acquisitions demonstrate Bollore's determination to keep the upper hand at Vivendi, the parent company of Universal Music Group (UMG), amid heightened expectation about a possible listing of the world's biggest music label.
Vivendi, which also owns France's leading pay-TV group Canal Plus, may make announcements about capital options for UMG next week when it reports first-quarter results.
Bollore had to cancel some of the voting rights attached to the Vivendi shares he already owned to avoid crossing the threshold of 30 percent, which automatically triggers a full bid for the remaining shares under French law, a source close to the matter said.
The 66-year-old businessman became a Vivendi shareholder in 2012 when he agreed to be paid in shares for selling 60 percent of his broadcast TV channels Direct 8 et Direct Star to Canal Plus.
He later bought additional shares on the market.
The billionaire stunned Vivendi shareholders at the group's general meeting on April 19 by unexpectedly stepping down as chairman and asking the board to appoint his son Yannick, 38, to replace him.
A week later, Bollore was placed under formal investigation over allegations his wide-ranging business empire corrupted officials to win port concessions in West Africa.
The group denies any wrongdoing.
(Reporting by Mathieu Rosemain; Editing by Mark Potter)
By Mathieu Rosemain