Lat. vice versa
The source of this expression is the Latin ablative absolute phrase vice versā, meaning something like ‘the position having been reversed’. It was formed from vĭce, ablative singular form of the feminine third declension noun vĭcis ‘alternation, turn; time, instance; arrangement, order’ (accusative wordform: vĭcem; source of Sp. vez), plus versā, feminine ablative singular of perfect passive participle versus of the verb vĕrtĕre ‘to turn’. The learned prefix vice‑ in English and Spanish is a cognate of this vice; it comes from Latin vice‑ ‘in place of’, e.g. Eng. vice-president ~ Sp. vicepresidente.
The cognates Eng. vertebra [ˈvɜɹ.tɪ.bɹə] and Sp. vértebra [ˈbeɾ.t̪e.βɾa] refer to ‘each of the series of small bones forming the backbone’ (COED). They come from Latin vertebra (accusative: vertebram), which meant ‘joint’, namely that which allows an articulation to bend and turn, a more general meaning than the modern words have, though the Latin word could be used for the vertebrae as well.
The Latin word vertebra was obviously derived from the root vert‑. The meaning ‘joint’, which refers to something that turns, makes that perfectly clear. To that root something was added that we recognize as the reflex of an early, Proto-Indo-European instrumental suffix which has been reconstructed as *-dʰrom and which had different reflexes in Latin words, primarily ‑bra and ‑brum, but also ‑bula and ‑bulum.
The nominative plural of vertebra in Latin was vertebræ, and thus the plural of the English word can be vertebrae also, pronounced [ˈvɜɹ.tə.bɹi], [ˈvɜɹ.tə.bɹeɪ̯] or [ˈvɜɹ.tə.bɹaɪ̯], sometimes written vertebræ, with the letter 〈æ〉, known as ash, originally a ligature of the letters a and e representing the Latin diphthong ae [ai̯]. English also allows the regularized plural vertebras. In Spanish the only option for the plural is the regularized vértebras.