This study examines which English vowel is matched with which vowel in Standard Mandarin in loanword adaptation, investigates the general patterns for and restrictions on vowel adaptation in Standard Mandarin loanwords, and determines which aspects of vowel quality is more carefully replicated than others. The results show that despite the seemingly high degree of variation, there are systematic patterns: (i) the front-back dimension is more faithfully replicated than height and rounding, (ii) deviation along the height dimension is tolerated but minimal, (iii) a rounding mismatch occurs mostly in adapting mid central/back vowels, and (iv) central vowels behave as if they are unspecified for or ambiguous between front and back. This study demonstrates how the grammar prioritizes which aspects to replicate in the loanword adaptation process and has implications for theories of loanword phonology in particular and feature theory in general.