NEW ORLEANS — U.S. Sen. David Vitter is taunting the latest group to claim ownership of the phrase “Who Dat,” sending a letter that tells the company to drop its trademark lawsuit or meet him in court.Vitter jumped into the fray over the popular Saints chant by sending two of his T-shirts to the owners of the company, “Who Dat? Inc.” The T-shirts, printed earlier this year when the NFL asked shop owners to stop selling Who Dat merchandise, say “Who Dat say we can’t print Who Dat! © TM 2010 David Vitter (just kidding)””You’re kidding, right? Claiming you own the legal rights to the term ‘Who Dat’?” Vitter wrote in his letter to the company.