Direct answer
An acoustic vent is optimized for sound transmission at microphone or speaker openings; a protective vent is optimized for airflow and pressure equalization in a sealed enclosure.
Different performance priorities
Acoustic selection focuses on frequency response, insertion loss, impedance, distortion and recovery after water exposure. Protective-vent selection focuses on airflow at a defined pressure, water-entry resistance, contamination control and pressure-response time.
Can one part do both jobs?
Sometimes a membrane can serve both an acoustic and pressure-relief role, but the trade-offs must be tested in the product. Many devices use separate parts because the required active area, location and performance targets differ.