This study draws on quantitative reasoning research to explain how secondary mathematics preservice teachers' (PSTs) modelling competencies changed as they participated in a teacher education programme that integrated modelling experience. Adopting a mixed methods approach, we documented 110 PSTs' competencies in Vietnam using an adapted Modelling Competencies Questionnaire. The results show that PSTs improved their "real-world-problem-statement," "formulating-a-model," "solving-mathematics," and "interpreting-outcomes" competencies. Showing their "formulating-a-model" and "interpreting-outcomes" competencies, PSTs enhanced their quantitative reasoning by properly interpreting the quantities and their relationships using different representations. In addition, the analysis showed a statistically significant correlation between PSTs' modelling competencies and quantitative reasoning. Suggestions for programme design to enhance modelling competencies are included.