Srpskoslovenski: the language written between Miroslav's Gospel and Vuk's reform
Miroslav's Gospel proved Serbian Cyrillic is centuries older than Vuk. It does not explain what medieval scribes were actually writing in it - not vernacular Serbian, but a distinct literary register with its own name: srpskoslovenski, the Serbian recension of Church Slavonic, which served Serbian lands for roughly six centuries before an imported Russian version of the same tradition helped produce slavenosrpski, and Vuk's reform finally retired the whole lineage.