When do a third party's rights to enforce the contract vest?

After you determine whether or not there is a third-party beneficiary, the next important question is, "When do their rights activate?" When rights become enforceable, they are said to have "vested." This is important, because once a third-party beneficiary vests, the promisor and promisee are no longer able to modify their agreement.

A third-party beneficiary can vest their rights in one of three ways:

(1) The beneficiary agrees to the terms of the promise in an appropriate way. For example, let's say Bob owes Sam $500. Next, Bob and Amy agree that if Amy pays Sam $500 to cover Bob's debt, Bob will paint Amy's house. Bob sends Sam an email saying, "Hey, just wanted to let you know that Amy has agreed to pay my $500 debt to you." In response, Sam says, "Ok, great!" - Here, Sam's acknowledgment and assent to Bob and Amy's arrangement causes Sam's rights to vest. Sam is now a creditor third-party beneficiary.

(2) The beneficiary brings a lawsuit to enforce the promise.

(3) The beneficiary reasonably relies on the promise. For example, imagine if Bob paid Sam to mow Amy's lawn. Sam agrees. Amy finds out about this arrangement and calls her usual landscaper and says, "Hey guys, I found out someone else is going to cut my lawn for me, so I won't need you for 2 weeks." Amy has relied on Sam's promise to Bob to her detriment and, as such, her rights will vest.

Note that once rights have vested, terms cannot change without consent from the third-party beneficiary.