Reese Witherspoon reesevince.jpg ...Four Christmases!

The Daily News is reporting that Reese Witherspoon and Vince Vaughn are clashing on the set of their new romantic comedy Four Christmases.

"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," the Daily News quotes a source as saying. "Sometimes Vince will be standing behind her and he has this look on his face that he just wants to kill her!"

Sounds tense - but then again, considering how Four Christmases tells the story of a married couple spending a Christmas with each of their four divorced parents, that tension might just add to the realism!