I realize it does sound kind of dumb, but what these others have suggested actually is the answer. I don't know where you live, but parking your car in full sun for a while should soften up that vinyl sufficiently that you can just peel the letters off with a fingernail or the edge of a credit card.
Whatever you use, though, make sure you don't scratch up the paint. Regardless, when you're done, you may want to use something like Meguiar's Scratch-X to polish the area.