This study investigates whether young heritage speakers, either simultaneous or sequential bilinguals, have limited vocabulary knowledge in their family language compared to matched monolingual counterparts and, if so, what factors help to account for this difference. These factors include age, age at emigration, length of emigration, frequency of heritage language use, and parents' attitude toward heritage language acquisition and maintenance. Thirty young Persian-English bilinguals (aged 6-18) living in New Zealand took a productive and a receptive vocabulary test. These tests were also administered to thirty monolingual speakers of Persian who were matched with the bilinguals for age, gender, number of siblings, and their family's socioeconomic status. Information about the heritage speakers' language use and their parents' attitude toward heritage language maintenance was collected through semistructured interviews. The results showed that the heritage speakers were outperformed by the monolinguals in both vocabulary tests, but the gap was wider in the case of the simultaneous bilinguals. Of the factors investigated, the parents' attitude was found to be the strongest predictor of the simultaneous bilinguals' vocabulary knowledge, which highlights the role parents can play in the heritage language development of their children if they are exposed to a majority language early on in life. On the other hand, the sequential bilinguals' vocabulary knowledge was associated mostly with age at emigration--the older the children were when they moved to the second language environment, the better their family language vocabulary tended to be.