The phase of vehicle extrication that determines the nature and extent of the overall situation is:

The phase of vehicle extrication that determines the nature and extent of the overall situation is size-up. This critical process involves assessing the scene to gather information about the incident, the vehicles involved, and any potential hazards. During size-up, firefighters observe and evaluate various factors, such as the condition of the vehicles, the number of victims, accessibility, and the presence of hazards like fuel leaks or electrical hazards.

A thorough size-up helps in developing an effective plan for extrication and ensures that resources are deployed appropriately. It's an essential step for ensuring safety and efficiency during the operation. By understanding the full scope of the situation, firefighters can make informed decisions on the best course of action for disentanglement and hazard control.