https://www.google.com/search?q=fourtteenth month's pay&rlz=...

https://en.wikipedia.org/wiki/Thirteenth_salary
...Some institutions practice adding a "fourteenth", "fifteenth", and successive bonuses for higher pay grades

The performance allowance is an additional full salary allowed to the tax officials, which was recommended under the World Bank-assisted tax reform programme to eliminate corruptio

Typically employees are paid monthly in Portugal and they also get two additional monthly salaries in a work year, just like in Spain. These extra salaries are usually known as 13º mês (beginning of December) and 14º mês (beginning of June).

In North America things are different. Vacation pay must be at least four per cent of the gross wages (excluding any vacation pay) earned in the 12-month vacation entitlement year or stub period (where that applies) for employees with less than five years of employment. Employees with five or more years of employment at the end of a 12-month vacation entitlement year or stub period (if any) are entitled to at least six per cent of the gross wages earned in the 12-month vacation entitlement year or stub period.
An employee’s contract of employment or a collective agreement may provide a greater right or benefit with respect to vacation time and/or pay.

I would keep it simple, but would specify that the bonus is given with respect to vacation - hence /vacation bonus/.

14th annual salary instalment