Accumuler is the term used in Canada, which has this same concept in labor law. The concept is this: as an employee, if you have the right to X number of vacation days per year of employment, then there has to be a way to calculate how many vacation days you have earned at any given point in the year, and how many you earn for partial years (e.g. if you start working on April 10 instead of on January 1).

Common calculation methods include: for every 80 hours worked, you get 1 vacation day (or 0.5 days or whatever the math requires to add up to the correct number of days per year). Or, for every month worked you accumulate 1 day (or however many). Then, of course, there need to be calculation rules for partial months worked, partial weeks, fractions of 80 hours, etc.

The source text here is saying that if you are placed in service after the first of the month, then as long as you're in service on or before the 15th of the month, you will accumulate vacation days as if you had been placed in service on the 1st of the month.



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Note added at 12 hrs (2019-07-03 20:09:06 GMT)
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PS: A Geneva-based source (so Swiss French) used a different turn of phrase: "Les droits à congé annuel sont acquis à raison de 2,5 jours ouvrables par mois de service actif."

But that's harder to work into your sentence as phrased. If the grammatical subject of the sentence is rights to vacation days, then those rights are "acquis à raison de..." (whatever the applicable calculation is). But the subject of your source sentence is the worker themselves (the flight attendant), so "accumuler" is the verb you want.