This study explored kanji handwriting behavior of Vietnamese learners of Japanese as a Foreign Language (JFL), focusing on single-kanji words with Kun-readings to minimize phonological overlap between Japanese and Vietnamese. Participants completed a real-time handwriting task using a stylus and tablet. The study analyzed writing latency, duration, and accuracy, examining how these were influenced by lexical knowledge, kanji frequency, visual complexity, and difficulty level. Results showed that higher lexical proficiency and more frequent kanji led to faster initiation times. Writing duration increased with visual complexity, as kanji with more strokes took longer to execute. Accuracy decreased for complex and difficult kanji (e.g., N2 level), especially among lower proficiency learners. Notably, learners with stronger lexical knowledge could better compensate for complexity during writing. These findings highlight the distinct cognitive and motor demands of kanji production and underscore the value of combining vocabulary exposure with structured handwriting practice in JFL instruction.