early 14c., "summon into court to prove a title," from Anglo-French voucher, Old French vocher "to call, summon, invoke, claim," probably from Gallo-Roman *voticare, metathesis of Latin vocitare "to call to, summon insistently," frequentative of Latin vocare "to call, call upon, summon," which is related to vox (genitive vocis) "voice" (from PIE root *wekw- "to speak"). Meaning "guarantee to be true or accurate" is first attested 1590s. Related: Vouched; vouching.

c. 1300, from Old French voieul (Modern French voyelle), from Latin vocalis, in littera vocalis, literally "vocal letter," from vox (genitive vocis) "voice," from PIE root *wekw- "to speak." Vowel shift in reference to the pronunciation change between Middle and Modern English is attested from 1909. The English record-holder for most consecutive vowels in a word is queueing.

Latin, literally "voice," which is the source of vocare "to call," from PIE root *wekw- "to speak."