The mastery of vowel production is central to developing vocal technique and may be influenced by language, musical context or a coach's direction. Currently, students learn through verbal descriptions, demonstration of correct vowel sounds, and customized exercises. Vowel Shapes is an interactive practice tool that automatically captures and visualizes vowel sounds in real time to assist singers in correctly producing target vowels. The system may be used during a lesson or as a practice tool when an instructor is not present. Our system's design was informed by iterative evaluations with 14 students and their vocal professor from the Eastman School of Music, University of Rochester. Results from an exploratory evaluation of the system with 10 students indicated that 70% of the participants improved their time to reach an instructor-defined target. 90% of the students in the evaluation would use this system during practice sessions.