Final answer:
The missing part likely causing vibrating brake pads are the anti-rattle clips, which stabilize the brake pads in the caliper assembly.
Step-by-step explanation:
When diagnosing a vehicle with vibrating brake pads, the parts that are most likely missing are the anti-rattle clips. These clips, also known as hardware or shims, are designed to keep the brake pads stable within the caliper assembly and prevent them from rattling or moving excessively. Without these clips, the brake pads may vibrate, which can cause noise and potentially affect braking performance.