During the migration period, Germanic tribes began to settle in Britain during the 5th century after Roman occupation came to an end.

[8] Most modern assessments estimate a document date of around 1200 or slightly later, from an original that was probably made around 1150 or sometime in the second half of the 12th century, presumably in Cornwall or by a Cornish speaker.

[9] The Vocabularium Cornicum is part of a composite manuscript known as MS Cotton Vespasian A XIV containing texts believed to have been made between the early 11th and late 12th century.

[8] As with Ælfric's Glossary, the lemmata are grouped thematically, usually with a Latin lemma followed by a Cornish translation equivalent, beginning with entries for God, heaven, angel, then elements of the Genesis creation narrative including star, sun, moon, the Earth and sea, and human beings.

The vocabulary continues with a range of subjects including parts of the body, ranks in the church hierarchy, family members, secular positions and class divisions, words for various kinds of professions and artisans and their associated tools, personality traits, illnesses and afflictions, legal terminology, the weather, times of the day, seasons of the year, colours, birds, fish, insects, domestic and wild mammals, herbs, trees, topographical features, architectural terms, household items, clothing, and food and drink, finishing with some adjectives and ending with the Cornish word for 'saddle'.

[10] Lemmata in the Vocabularium Cornicum are conventionally labelled by the numerical identifier they are assigned by Eugene Van Tassel Graves in the PhD dissertation The Old Cornish Vocabulary.

[20] Until Edward Lhuyd established that the language was Old Cornish in Archæologia Britannica in 1707, the text was thought to be Welsh, and in the Cotton library was originally classified as Vocabularium Latino-Cambricum (Latin-Welsh Vocabulary) and was inscribed with the text Vocabularium Wallicum (Welsh Vocabulary).

[10] As Old Welsh, Old Breton, and Old Cornish were very similar at this time, many of the glosses would have been indistinguishable in all three languages, and Alderik Blom estimates that around 35% of the entries would have been spelled almost identically in Cornish and in Welsh, without even taking into account the considerable number of words that would have exhibited only minor vocalic differences.

[24][25] However, by the time of the Vocabularium Cornicum, the orthography, which Jackson describes as "chronologically more advanced than that of any other [Old Cornish] document",[23] shows the increasing influence of Old English scribal practices, such as the use of the graphemes thorn (Þ, þ), eth (Ð, ð), and wynn (Ƿ, ƿ).