<p>This article examines how multilingual higher education (HE) can move beyond treating generative Artificial Intelligence (AI) as a writing instrument toward Writing-With AI as a postdigital pedagogical stance. It contributes primarily to scholarship on critical AI literacy and multilingual academic writing, while drawing on postdigital pedagogy and duoethnography to frame the conditions under which AI-mediated writing now occurs. We write as two Vietnamese doctoral researchers located across two asymmetrical sites: undergraduate English-medium writing classes in the Mekong Delta and postgraduate academic writing spaces at a European university. Using duoethnography, we analyse teaching journals, writing encounters, reflective memos, author dialogues, draft versions, and AI-mediated writing traces. Our analysis identifies three recurring tensions: fluency may erase stance, English academic output may conceal multilingual process, and polished claims may weaken evidentiary judgement. To address these tensions, we conceptualise Writing-With AI through a 4R engine: relational, reflexive, recursive, and re-forming. The article translates this framework into modest pedagogical practices, including AI-use prefaces, ghost logs, bilingual process notes, kept-rough sentences, corroboration sprints, and companionship notes. It contributes a situated account of AI-mediated writing in multilingual HE and offers learning designs for preserving writerly presence, multilingual process, and critical judgement in postdigital academic writing.</p>