Well, it wasn’t exactly vernacular. Old Church Slavonic was a language which was intelligible to the various Slavic nations, but was still markedly distinct from any of the actual vernacular tongues. It had a greatly expanded lexicon, mainly adapted from Greek, otherwise it would be unable to refer to most of sacral concepts, its syntax was notably non-Slavic at some points, since the translators often mimicked the constructions in the original Greek source etc. It was a liturgical language, even though it was intelligible to the people, at first. When the Proto-Slavic areas fell apart and developed into medieval versions of the modern languages, the churches did not switch from CS to the vernacular. Neither does the Greek church use Modern Greek in liturgy, but the ±2000 years old Koiné.
And even then, so what? Use of “vernacular” is proper to the Eastern Church, use of Latin to the Western Church. We have no obligation to mimic the East at every turn; we have our own patrimony, even if it was forgotten to a great extent.