This study examines how Vietnamese adult learners of Australian English modified their criticisms in a peer-feedback session. Data were collected from three groups of learners (12 high beginners, 12 intermediate, and 12 advanced), via a conversation elicitation task, a written questionnaire, and a retrospective interview. L1 and L2 baseline data were collected from two respective groups of 12 Vietnamese native speakers and 12 native speakers of Australian English, via the same conversation elicitation task and questionnaire. Results showed that learners, regardless of their proficiency levels, tended to under-use modality markers, especially internal modifiers. A number of factors might have influenced this pragmatic behavior: incomplete L2 linguistic competence, L1 transfer, and cognitive difficulty in spontaneous language production. The study also found evidence of an acquisitional order for criticism modifiers: learners tended to acquire lexicalized modifiers before they acquired grammaticalized modifiers. This finding lent support to Meisel et al.'s (1981) Complexification Hypothesis, which holds that the order of acquisition of L2 forms is dependent upon their structural complexity and the processing demands involved; thus syntactically complex structures, which are also more cognitively demanding, are usually acquired later than simpler structures which require a minimum of processing capacity.