Numerous empirical studies and syntheses have explored technology-mediated task-based language teaching (TMTBLT) (for example, Chong & Reinders, 2020; Kim & Namkung, 2024; Lai & Li, 2011; Ziegler, 2016), with findings highlighting the affordances of technology for interaction and its associated benefits. However, few studies have targeted program development and evaluation for task-based online curriculums for Less Commonly Taught Languages (LCTLs) (Bryfonski & McKay, 2019). To address this gap, the current study uses action research (Burns, 2010) to report a teacher-researcher's experience in creating, implementing, and evaluating TMTBLT materials for a beginning Vietnamese language class at an American university. Action research was used as the methodological framework through two iterative cycles (that is, two consecutive semesters) conducted with two mixed cohorts of heritage language learners and second language learners. Results highlighted the benefits of AR for bottom-up curriculum development and demonstrated significant gains in learners' speaking and writing proficiency over time. Findings also suggest learners had overall positive perceptions of TMTBLT as a pedagogical approach.