The influence of the Vernacular Orthography used in Old Russian birch bark letters on standard book writing in Old Rus’ is traditionally regarded as insignificant, being merely associated with single inclusions. According to our observations, in the 13th century, during the period of the most active use of graphics with a variation of letters о – ъ and е – ь in vernacular writing, this influence in individual handwritings may have been more significant. In the 13th century handwriting of the Gospel from the RGADA collection (manuscript collection, no. 816), the variation of the letters о – ъ and е – ь is represented by hundreds of examples. Their detailed analysis allows us to describe the influential features of Vernacular Orthography on the book spelling system in this period – the normalization mechanisms employed by book scribes to eliminate vernacular effects, along with the most favourable positions for the penetration of these effects into book spelling itself. The article shows that general data on the reflexes of Vernacular Orthography in the manuscript under consideration gloss over the noticeable contrast in the individual positions relevant from the spelling point of view. A more significant influence of the graphic effects е = ь, о = ъ on book spelling occurred in the presence of correlating forms within it, differing in a pair of letters (the flexion of the present tense -еть/-ете, forms of different genders like правъ / право, forms of participle and aorist like падъша / падоша). In some morphemes, the share of vernacular spelling positions in the manuscript may be one third or more than half of all the examples of a given morpheme.