Tony Taylor, 34, who will be sentenced Dec. 14, said he was not promised any specific sentence in return for his cooperation with the government.
Taylor, of Hampton, Va., entered his plea in U.S. District Court to conspiracy to travel in interstate commerce in aid of unlawful activities, and conspiring to sponsor a dog in an animal fighting venture.
Vick, the star quarterback for the Atlanta Falcons, and two other co-defendants pleaded not guilty last week to similar charges.
The offenses carry a maximum punishment of five years in prison and fines of up to $250,000.
Prosecutors claimed in a July 17 indictment that Taylor found the Surry County property that Vick purchased and used as the site of "Bad Newz Kennels," a dog-fighting enterprise. Taylor also allegedly helped purchase pit bulls and killed at least two dogs that fared poorly in test fights.
According to the 18-page indictment, the dog-fighting ring executed underperforming dogs by drowning, hanging and other brutal means. It alleges that the fights offered purses as high as $26,000.
The grisly details outlined in the indictment have fueled protests and public outrage against Vick. NFL commissioner Roger Goodell has banned Vick from the Falcons' training camp while the league investigates.
Vick and Purnell A. Peace, 35, of Virginia Beach, Va., and Quanis L. Phillips, 28, of Atlanta, are scheduled for trial Nov. 26. They remain free without bond.
Vick, meanwhile, lost another endorsement deal over the weekend. Upper Deck has removed all Michael Vick autographed memorabilia from its online store and will remove the indicted quarterback's trading card from NFL sets that are scheduled to be released in October.
The announcement by the San Diego County company came a day after Nike suspended its lucrative contract with Vick and Reebok took the unprecedented step of stopping sales of his No. 7 jersey. Another trading card company, Donruss, announced it was pulling Vick's likeness from any new packs.
"Of course we appreciate the fact that Mr. Vick is innocent until proven guilty, but the allegations alone have resulted in an outpouring of very strong emotion within our organization and among the collecting community," Kerri Stockholm, Upper Deck's director of marketing, said in a statement. "We believe collectors will agree and support this decision as being the best course of action for our football business."
Items pulled from the online store include autographed footballs, helmets, jerseys and the company's line of "Breaking Through" pieces. His card will be pulled beginning with the Ultimate Collection Football set.
A large crowd gathered outside the Georgia Dome Sunday afternoon to show their support for Vick, reports CBS Atlanta affiliate WGCL.
"The American Judicial system is innocent until proven guilty," said Selena Adkins.
"If he's not given a fair shot, I won't be a season ticket holder," said Roderick Green.
"I say give Michael a fair chance. They act like he's already been convicted," added Dewanna McGruder.