Final answer:
Bathroom fixtures made of porous materials like unsealed grout or plaster, as well as plastic shower curtains and certain paints, are most vulnerable to smoke residue because they absorb smoke particles easily.
Step-by-step explanation:
The bathroom fixture surface that is most vulnerable to smoke residue typically includes any porous materials such as unsealed grout or plaster. These surfaces absorb smoke particles more readily than non-porous surfaces like glazed tile or porcelain. Smoke residue can cling to the rough or uneven surfaces, making them harder to clean. In bathrooms specifically, fixtures like plastic shower curtains or certain types of painted walls can also be vulnerable. It's important to address smoke residue promptly to prevent lasting damage and odor.