The rapid advancements in artificial intelligence have facilitated the incorporation of AI models such as ChatGPT into language education, offering learners insightful feedback and support in writing tasks. This study aims to explore learning strategies employed by Vietnamese undergraduates, the obstacles they encounter, their methods for overcoming these hurdles while utilizing ChatGPT to assist with writing English essays, and their perceptions of the effective integration of ChatGPT for writing instruction. With a mixed-methods design, the study involved 61 Vietnamese undergraduates, employing surveys, semi-structured interviews, and simulated writing tasks to collect data. While quantitative data were analyzed descriptively, thematic analysis was applied to qualitative findings to obtain deeper insights into students’ experiences with the Chatbot. The results showed that learners mostly employed social, compensation, and memory strategies. Moreover, they often encountered challenges with ChatGPT’s content and communication with the Chatbot, which necessitated their attempts to refine prompts and be more critical of ChatGPT’s contents. Additionally, in order to appropriately integrate ChatGPT into writing instruction, the participants perceived that more ChatGPT training and teacher support were crucial. These findings provide valuable resources for educators and learners seeking to incorporate AI into language instruction, providing them with insights into learners’ preferred strategies for AI-assisted writing, potential difficulties linked with its employment, and viable approaches for utilizing ChatGPT in writing instruction.