Abstract We report preliminary results of a mixed-methods longitudinal study of academic writing by graduate students writing in English in Vietnam.  The research involves Vietnamese EFL teachers (n=62) taking a 2-year MA Linguistics (TESOL) program delivered in hybrid mode.  The program, jointly taught by an American private university and a large public university in Vietnam, uses an innovative scaffolded thesis model developed by the researchers. In program entry surveys, vocabulary was mentioned most often as a significant challenge for academic writing. We compared the vocabulary that our students used in early and late drafts of their theses against two reference corpora: The graduate level linguistics subcorpus of the BAWE corpus of student academic writing and COCAA 2010-12.  Data from the first drafts of the introduction, discussion and conclusion sections shows the Vietnamese students compare favorably with both native and non-native students in the BAWE subcorpus in terms of lexical density measured as type to token ratio, but their use of both discipline-specific and “core academic” words is more limited in scope. Between drafts, the overall length of writing expanded by approximately 17% and students collectively added 302 word types including 25 core academic and 11 discipline specific. It is clear from our research that vocabulary cannot be disentangled completely from other aspects of academic writing such as lexical bundles, syntax, and cohesion. Our research will enhance writing pedagogy in EAP with suggestions for developing targeted and contextualized instruction in using academic vocabulary, and it will be of particular interest to instructors of advanced students writing in an EFL environment.