This paper shows that morphosyntactic vagueness exists between several minor morphosyntactic categories in Mandarin (modal, protasis connective and types of classifier): these categories share formal distributional properties and functional similarities such that their morphosyntactic distinctions become neutralized in some contexts. Consequently, bidirectionality rather than unidirectionality may characterize these categories diachronically. This paper proposes that any direction of change that is motivated by morphosyntactic vagueness should be regarded as regular and systematic, even if it is not unidirectional, and identifies some directions for future research on morphosyntactic vagueness in diachrony.