New York-based Third Point has been invested in Paris-based Vivendi for some time, the person said, requesting anonymity because the matter is private.
A Vivendi spokesman on Monday said, "The company has yet to see any evidence of a stake being built," in response to a question about the Third Point holding. A Third Point representative declined to comment.
Third Point's stake was first reported by Bloomberg.
Vivendi has become popular with certain investors ever since it announced plans to spin off 60% of Universal. Earlier this month Ackman said his blank-check company Pershing Square Tontine Holdings is planning to buy 10% of Universal before the planned spinoff.
Vivendi, controlled by French tycoon Vincent Bollore, last month set out a plan to list Universal, the world's largest music group, by Sept. 27.
Investors will have a chance to vote on the proposed spinoff later this month and some have already expressed reservations.
Activist fund Bluebell Capital Partners and investment fund Artisan Partners have raised concerns about the spinoff, decrying the transaction as unfavorable to some investors.
Glass Lewis and ISS, the world's two biggest proxy advisers,
have recommended Vivendi shareholders vote in favor of the
Third Point is trying to evaluate what Ackman's proposal would mean for Vivendi shareholders and has not commented publicly about the matter.
Loeb and Ackman, both prominent New York hedge fund managers, have crossed on other investments, including Herbalife and United Technologies, for example, and sometimes taken differing approaches.
Loeb has had his own blank check company and invested in a number of others, according to letters to his investors and regulatory filings. He did not invest with Pershing Square's Tontine Holdings, which counts other prominent hedge funds as investors.
($1 = 0.8247 euros)
(Reporting by Eva Mathews, Svea Herbst-Bayliss and Gwenaelle Barzic; Editing by Ramakrishnan M. and Steve Orlofsky)
By Svea Herbst-Bayliss