Cognitive neuroscience has gained significant insight into the mechanisms underlying the mental lexicon and their impact on second language vocabulary learning. However, relatively little effort has been put into understanding how these mechanisms may impact instructional practices. We attempt to bridge this gap. Towards that end, we first describe three main properties of the mental lexicon: words have distinct form and meaning representations; these representations are organized in multilingual network structures organized by similarity; and the representations differ in strength. Next, we translate the properties into a visual framework that we use to reflect upon how this knowledge affects three important choices in vocabulary instruction: composing word lists, selecting an instruction method, and consolidating newly acquired words. Lastly, we discuss opportunities for both science and practice, highlighting the importance of improving the ecological validity of scientific models and theories, and of developing instructional methods grounded in cognitive science.