In the event of ventilator malfunction, what is the immediate step a flight paramedic should take?

In the event of a ventilator malfunction, the priority is to ensure that the patient continues to receive adequate ventilation. Manually ventilating the patient using a bag-valve-mask device is the immediate step to take because it can quickly provide positive pressure ventilation while addressing the issue with the malfunctioning ventilator. This intervention helps maintain oxygenation and carbon dioxide removal, which is critical for the patient's stability.

The other options, while important in a broader context of patient care, do not directly address the immediate need for ventilation. Assessing vital signs is necessary but should come after ensuring the patient is ventilated appropriately. Changing the ventilator settings may not be effective if the problem is a malfunction that wouldn’t be resolved by adjustments. Calling for assistance could be useful, but it doesn't provide the immediate support that manual ventilation does in a critical situation. Thus, using a bag-valve-mask device is the essential first response to maintain patient safety and physiological needs.