Hollywood slang for a
film designed to rocket a new discovery to movie stardom, often from another area of showbusiness.
Singer Mariah Carey hopes the her upcoming film "Glitter" might help prove her as a serious actress, as has Madonna with... well... pretty much all of her bombs. Similarly, the bomb "A Knight's Tale" was meant to propel Heath Ledger into a teen idol position.
As vehicle films often focus on transforming the star from one art to another, they often focus on his/her explointing existing talent, leaving other aspects of moviemaking such as plot behind.
Generally, vehicles are about as successful as putting a mechanic in an operating room - just because he knows how to put together one system, doesn't mean he can't nail together another.