The event is scheduled for Dec. 29.
Belcher gets a chance to exact revenge against Okami, the first man to defeat him inside the Octagon.
The two first met in August 2006 at UFC 62. It was the promotional debut for each fighter. Okami (27-7) won the contest by unanimous decision.
After dropping two fights in a row, including a second-round TKO to champion Anderson Silva at UFC 134 in August 2011, Okami rebounded on Aug. 11 with a second-round TKO of Buddy Roberts.
Belcher put himself in the 185-pound title conversation on May 5 with a first-round TKO of Rousimar Palhares.
He was scheduled to fight Vitor Belfort in October at UFC 153 in Rio de Janeiro, Brazil. That bout was scrapped when Belfort agreed to face light heavyweight champion Jon Jones on Sept. 22 at UFC 152 in Toronto.
Belcher (18-6) is ranked seventh among middleweights by ESPN.com.
The promotion suspended Leben after the fighter failed a UFC 138 postfight drug test. He tested positive for oxycodone and oxymorphone after his second-round TKO loss to Mark Munoz on Nov. 5.
It was Leben's second drug-related suspension by UFC.
"I like Chris and I want him to do well, but based on his actions, he's been suspended for one year," UFC president Dana White had said. "If he needs professional help, we are going to be there for him.
"We want to see him succeed not only in the Octagon but in his personal life."
Leben has a professional mixed martial arts record of 22-8. Vemola is 9-3.