Final answer:
Not smoking is a preventive measure for vulvar cancer, as smoking is a risk factor for related cancers like cervical cancer.
Step-by-step explanation:
Among the options provided, not smoking is a preventive measure for vulvar cancer. Smoking is a known risk factor for cervical cancer, and by extension, it is understood that reducing or eliminating smoking can also reduce the risk of developing other related cancers, including vulvar cancer. While avoiding exposure to HIV can support overall health, it is specifically noted for its link to a weakened immune system, increasing the risk of cervical cancer, and keeping the immune system strong could indirectly help in preventing vulvar cancer too. However, the most direct measure from the options listed would be not smoking.