A vociferous person is loud, noisy, and clamorous; a vociferous crowd is characterized by unrestrained yelling.
A voracious person or animal, meanwhile, devours food ravenously; a voracious reader has an insatiable desire for books, magazines, and other reading materials. In short, although the two words appear similar, they apply to very different types of behavior. Through word-swapping, they get confounded. “Vociferous” is sometimes misused for “voracious” — e.g. (the word in red below is incorrect, it should be the one in green in the parenthetical aside):
“Monsignor Field [is] a nurturing mentor, with a depth that allowed him to be at once an aficionado of the opera, a vociferous [read ‘voracious’] reader and a fan of basketball.” Susan Todd, “Rev. William Field, 84, a Priest’s Priest,” Star-Ledger (Newark), 4 Dec. 2000, at 67.
“Atlanta’s vociferous [read ‘voracious’] appetite for water isn’t likely to wane anytime soon.” “Protecting Our Water,” Greenville News (S.C.), 19 Feb. 2002, at A6.
The opposite error is somewhat less common — e.g. (the word in red below is incorrect, it should be the one in green in the parenthetical aside):
“Ironically, despite Olson’s voracious [read ‘vociferous’] opposition to federal special prosecutors, one of his oldest, best friends is Kenneth Starr, the Clinton special prosecutor.” Chuck Goudie, “Bush Attorney Held Keys to Operation Greylord,” Daily Herald (Chicago), 15 Dec. 2000, at 10.