Can steam from my rice cooker's steam vent sanitize sponges?

If boiling, microwaving or washing sponges in the dishwasher or cloth washer does not disinfect them, steaming them will not, especially in a rice cooker vent where the steam quickly loses temperature.

Throw them away and replace them regularly or, if feeling ecologically conscious, don't use sponges.

Steam from rice cookers also carries a certain amount of rice starch which would get deposited on the sponges in the situation you describe.

Rice starch will provide an additional food source for bacteria, likely encouraging their growth.