This study aims to provide an understanding of factors that might affect three-level credit constraints of small and medium-sized enterprises (SMEs) in Vietnam, using data from the Vietnam’s Manufacturing SME Survey database. We apply a random-effects generalised ordered probit model to accommodate two types of heterogeneity: unobserved individual heterogeneity and the heterogeneous effect of independent variables across three constraint categories. Results show that several firm and owner characteristics are in play. Several of these are indeed heterogeneous, such as varying across constraint categories, a new finding in the literature. A key contribution of this study is the use of a trinary rather than the usual binary approach to measure and more fully understand determinants of credit constraints in the case of small and medium-sized enterprises in Vietnam.