。
It's in my contract that I get one month of vacation every year.
Literal
Every-year 1-month's vacation [object-を] receive thing [subject-が] contract's clause [into-に] has-become.
~ことになっている ('it has been decided that X,' 'the arrangement is that X') expresses an externally set rule or agreement. Passing through ことになる gives the flavor of 'it's arranged/decided,' not the speaker's choice. Common in describing rules, contracts, and customs. 条項 ('clause, stipulation') is a formal legal term.