Final answer:
In law, an intended beneficiary's rights vest when the terms for vesting are met, such as consent by the promisor and sometimes by the beneficiary giving consideration or relying on the promise.
Step-by-step explanation:
The rights of an intended beneficiary of a promise, in legal terms, vest when the conditions for vesting have been met, which typically include the beneficiary becoming known to the promisor and consented to by the promisee, and sometimes by the beneficiary giving consideration or just relying on the promise in a substantial way. The vesting may occur when the contract is made or at a later point specified by the terms of the contract or the intentions of the parties involved. This concept relates to the third party beneficiary rule in contract law, which allows a person who was not a party to the contract to have enforceable rights if the contract was made for their benefit.