This study investigates the acquisition of wh-word order in Vietnamese by Japanese learners of Vietnamese as a second language (L2). Although both Vietnamese and Japanese follow a wh-in-situ structure, differences in syntactic flexibility present unique challenges for Japanese learners. A 9-point acceptability judgment task, followed by a correction phase, was conducted with 40 Japanese learners of Vietnamese and 30 native Vietnamese speakers. Participants rated the acceptability of wh-word placements in different positions relative to the main sentential components including the subject and the verb, on a scale from 1 (completely unacceptable) to 9 (highly acceptable). This approach provided a nuanced assessment of perceived acceptability across different syntactic configurations. The findings indicate that while Japanese learners generally align with native Vietnamese speakers on the basic placement of wh-words, especially tại sao ‘why’ and khi nào ‘when’ before the subject and như thế nào ‘how’, ở đâu ‘where’, cái gì ‘what’, and ai ‘who’ after the verb,challenges arise with non-standard placements, such as như thế nào ‘how’ before the subject and khi nào ‘when’ in various tense-related positions. These difficulties are likely influenced by L1 Japanese, where syntactic scrambling allows more flexible wh-word positioning. The study underscores the need for targeted L2 instruction that addresses these syntactic differences, with an emphasis on reducing L1 interference and clarifying marked structures in Vietnamese to enhance language acquisition.