The owners are expected to vote on the Raiders' proposed relocation Monday at around 2 p.m. ET, according to Schefter.
Oakland mayor Libby Schaafis asking the owners to delay voting in order to give her city a chance to negotiate with a small group of owners to complete a stadium deal at the Coliseum site.
The letters were given to owners Monday morning in Phoenix before a vote is expected on allowing the Raiders to relocate.
She also asked for owners to conduct a secret ballot on the vote.
Commissioner Roger Goodell told ESPN's Sal Paolantonio that the voting outcome on the Raiders' move to Las Vegas will be "positive."
"I think we will have a vote, and I think we will have a positive vote. I think we are in pretty good shape," Goodell said.
City, county and state officials have pledged $750 million in public funding toward a new stadium in Las Vegas.
Raiders owner Mark Davis needs at least 24 "yes" votes from the 32 owners to leave the city his franchise has called home for 45 of its 58 seasons, including the past 22.
Cowboys owner Jerry Jones also said Sunday that Monday, the start of the NFL's annual spring meeting in Phoenix, is "going to be an exciting day for Vegas."
Goodell's comment to Paolantonio comes after the league sent a letter Friday to Schaaf that stated a last-ditch proposal from the city was not a "viable solution."
ESPN's Kevin Seifert and The Associated Press contributed to this report.
Raiders' Vegas move just a matter of time
Field Yates says the vote to move the Oakland Raiders to Las Vegas has near unanimous support by NFL team owners.