Lannan accepted a minor-league assignment rather than become a free agent on April 19. He took the time between then and now to attend to a family matter.
He had a 15.75 ERA in five relief appearances for the Mets this season.
He had been part of the fifth-starter competition in Spring Training, but later switched to pitching in relief for the first time in his career.
Lannan will initially pitch in the bullpen for Vegas, but could also find himself moved back to the starting rotation (ESPN, Apr. 27).