A more thorough explanation:

Definition: To weaken or cancel out the effect of something else. For example, a law that vitiates a contract makes at least one part of the contract invalid.

Example: A company signs a contract with a supplier to purchase a certain amount of goods at a set price. Later, a new law is passed that puts a limit on the price that can be charged for those goods. This law vitiates the part of the contract that sets the original price, making it invalid.

Explanation: The example shows how a new law can vitiate a previously agreed-upon contract. The law weakens or cancels out the effect of the contract's price provision, making it no longer valid. This can cause problems for both parties involved in the contract.