Filming on the festive romantic comedy is rumoured to have been disrupted because of the stars' conflicting attitudes to work.
Reports claim that Witherspoon is angry at Vaughn's relaxed approach and refusal to rehearse their scenes.
A source told the New York Daily News: "Vince rolls onto set in the morning looking like he just came in from a night out, while Reese will arrive early looking camera-ready.
"Then Reese tries to force Vince into blocking out each scene and running through their lines as Vince tries to convince her that he's an ad-libber and wants to play around and see where the scene goes."
The source added: "She's a one-take perfectionist and Vince likes to try it a few different ways. Sometimes Vince will be standing behind her and he has this look on his face that he just wants to kill her!"
The pair are also producing the film, which is scheduled for release next Christmas.