This study examines the adaptation of French vowels in Vietnamese focusing on adaptation patterns that seem to defy a straightforward analysis based on native phonotactic restrictions or comparison of phonetic input-output similarity. A proper analysis requires reference to knowledge of the input language phonology. In the first case study, we observe that Vietnamese adapters extend the French phonotactic tendencies, i.e., Loi de Position, to loan adaptation productively. Such “intrusion” of L2 phonology knowledge may arise when phonetics underdetermines the adaptation and the adapters look to their knowledge of L2 phonology to arrive at adaptation. It is also notable that the L2 knowledge employed in adaptation is not native-like as the adaptation is not always isomorphic to the French input. In the second case study, the contrast of L2 phonology (/ʁ/ vs. /k/) is neutralized due to an L1 phonological restriction (i.e., no /ʁ/ in Vietnamese coda) but the Vietnamese adaptation systematically retains the contrast in the quality and length difference in the preceding vowel. There is plausible phonetic motivation for this adaptation pattern, but phonetically faithful mapping underdetermines the attested adaptation pattern, and reference to knowledge of L2 phonological contrasts is necessary. These findings illustrate the complexity of the loanword adaptation process, where a variety of different factors including L1 phonological restrictions, phonetic similarity, and L2 phonological knowledge, interact to affect adaptation.