A cross-sectional study of the performance of groups of Vietnamese learners is reported with focus on how they deal with the expression of number in English (singular/plural; definite/indefinite) through a cloze exercise and a translation excercise. This research investigates the hypothesis that some NP environments facilitate the distinction between singular and plural, count and mass, and that the context in which a noun is used can provide positive clues to the choice of number in nouns. It has been found that transfer of Vietnamese NP structures into English occurred where the NP environment was not obviously countable or uncountable, i.e., it has no conspicuous structural signals for number determination. Transfer was also found where an NP was taken from its context. The analysis of learners' errors gives some insight into ways in which the teaching of the number expression can be made more effective and beneficial for Vietnamese learners. A number of activities were suggested, which enable the teacher to exploit the advantages of NP environments to convey the syntactic-semantic properties of number to learners. Communicative practice of NP structures (e.g., in a conversation or a role play activity) can make learners aware of different aspects of the number expression in English. It is argued that the pragmatic aspect of the number expression is most important as in use, the syntactic and semantic properties of the category of number are unified to achieve communicative purposes.