Bringing in a crawl space dehumidifier will condition the inside air. Closing off your vents will prevent outside air from entering, but the dehumidifier will remove moisture from the air that’s already in your crawl space. This will help combat existing humidity problems and keep your insulation dry.
As mentioned earlier, you may or may not need a full encapsulation depending on the condition of your crawl space. Nonetheless, ignoring any problem signs, no matter how big or small, could lead to more damage and even more expensive repairs.