The aim of this intervention study was to examine the effect of Robust Vocabulary Instruction in a Swedish second grade regular classroom setting, to evaluate to which degree a rich vocabulary instruction is beneficial for all students’ vocabulary growth. Sophisticated target-words (tier 2 words) from a children’s book read aloud, were instructed to 24 children in the experimental class, whereas 20 children in the comparison class heard the same story but received no instruction. Students in each classroom were divided into two sub-groups based on the results of a standardized vocabulary test, group A with an estimated vocabulary knowledge below age-level, and group B with an estimated vocabulary knowledge within age-level. To assess their knowledge of the target-words, all 44 pupils were tested pre and post intervention. Both groups of children in the experimental classroom made significantly higher results post intervention. Gains were largest in experimental group B. In experimental group A the effect of the intervention varied considerably. Four pupils out of ten in experimental group A made little or no progress. The results suggest that some children in language vulnerability might not respond to vocabulary instruction as expected in a whole classroom setting, which calls for a differentiated and multi-tiered approach, with a higher level of support for these children in small groups or one-to-one design.