Phrase borrowed (more or less) from TV's "Wheel of Fortune", indicating disrespect for someone's mental abilities. Since generally the contestants on Wheel aren't exactly
Einstein, telling someone to buy a
fucking vowel is about equivalent to "Get a clue,
moron!"
You think the War on Drugs is working? Buy a
fucking vowel!
President Bush wants to
help the middle class, eh? Maybe he should buy a
fucking vowel with his family fortune.