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In a metaphor, the vehicle is the image or idea that represents the subject. In other words, the vehicle is what is giving us a new way to imagine the tenor or subject of the metaphor.
Here's an example:
George Clooney is a total angel.
The tenor (or subject) of this metaphor is "George Clooney," while the vehicle is "angel."
Here's another example:
I devoured the latest Hunger Games book.
The tenor of this metaphor is "reading" (a word not used here) and the vehicle is "devoured." See, sometimes the tenor might not be so obvious. But the vehicle is always there. In this case, the writer is comparing reading to eating—devouring, to be exact.