Vowel space (VS) measurements can provide objective information on formant distribution and act as a proxy for vowel production. There are a number of proposed ways to quantify vowel production clinically, including vowel space area, formant centralization ratio, and vowel articulation index (VAI). The stability, reliability, and sensitivity of three VS measurements were investigated in two experiments. Stability was explored across three inter-recording intervals and challenged in two sensitivity conditions. Data suggest that VAI is the most stable measure across 30 s, 2 h, and 4 h inter-recording intervals. VAI appears the most sensitive metric of the three measures in conditions of fatigue and noise. These analyses highlight the need for stability and sensitivity analysis when developing and validating acoustic metrics, and underscore the potential of the VAI for vowel analysis.