PERSON B: "Can I buy a vowel?"
1) LSD, also known as acid (A)
2) MDMA, also known as ecstasy (E)
As A and E are vowels, to "buy a vowel" is to purchase one of these drugs. The phrase is derived from the popular "Wheel of Fortune" television game show, but its raver connotations were spread by the movie "Groove."
Dealer: "All right. A or E?"