This study examines how dominance and proficiency relate to Spanish heritage speaker vowel productions. Participants' normalized vowel measurements were compared to nonheritage native speakers of Spanish and English using the Pillai score, an output of Multivariate Analysis of Variances (MANOVAs) that allows comparisons across distributions of two or more dependent variables. With Pillai scores as the dependent variable, we created two multiple regression models for each language, one with factors related to dominance, one with factors related to proficiency. We use commonality analysis (variance partitioning) to determine the unique and shared contribution of each variable to the regression models. The results showed different patterns of unique and shared variance across English and Spanish for the factors related to dominance and also for proficiency. Given this, we maintain that it is important to preserve dominance and proficiency as separate but related constructs when considering heritage speaker linguistic data.