Federline, 35, and Prince, 30, are also planning to marry in the same weekend, reported Us magazine.
"She said yes! Kevin told her he was going to Vegas for a job, but he surprised her by proposing and (then plans on) marrying her while they are there.
"Victoria really wanted this for a long time so she's gotta be blown away. She's proven herself with him from day one. Victoria is an awesome girl," a source said.
The couple have been dating since 2008 and are parents to their 1-year-old daughter, Jordan Kay. Jordan is with them in Vegas.