This is what David Sear's website says on the subject
"
Vota (plural of
votum). A vow made to a god in order to obtain a divine favour stipulated in advance. The granting of the request obliged the vower to fulfil
his promise. This usually took the form of a sacrifice to the deity or an offering to
his (or her) temple. Public
vota in Imperial times were normally for the welfare of the emperor over a stated period of time (five or ten years) and were regularly undertaken (
vota suscepta) and hopefully paid (
vota soluta). Sometimes they were more specific, relating to the safety of the emperor on a particularly hazardous journey or
military campaign, or the current state of
his health. The undertaking and fulfillment of these public vows was frequently recorded on the coinage and in
the late Empire especially may provide useful evidence for the chronological arrangement of issues."
The
inscription on your (very nice) coin,
VOT X MVLT XX means "Ten year vows redeemed, many twenty year vows undertaken". As the combined reign of Julian as
Caesar and
Augustus was less than ten years he must presumably have made
his vows before becoming emperor!