Mandarin Chinese has dental, palatal, and retroflex sibilants, but their contrasts before [_i] are avoided: The palatals appear before [i] while the dentals and retroflexes appear before homorganic syllabic approximants (a.k.a. apical vowels). An enhancement view regards the apical vowels as a way to avoid the weak contrast /si-ɕi-ȿi/. We focus on the dental vs. palatal contrast in this study and test the enhancement-based hypothesis that the dental and palatal sibilants are perceptually less distinct in the [_i] context than in other vowel contexts. This hypothesis is supported by a typological survey of 155 Chinese dialects, which showed that contrastive [si, tsi, tsʰi] and [ɕi, tɕi, tɕʰi] tend to be avoided even when there are no retroflexes in the sound system. We also conducted a speeded-AX discrimination experiment with 20 English listeners and 10 Chinese listeners to examine the effect of vowels ([_i], [_a], [_ou]) on the perceived distinctiveness of sibilant contrasts ([s-ɕ], [ts-tɕ], [tsʰ-tɕʰ]). The results showed that the [_i] context introduced a longer response time, thus reduced distinctiveness, than other vowels, confirming our hypothesis. Moreover, the general lack of difference between the two groups of listeners indicates that the vowel effect is language-independent.