When full grown, the flea caterpillar makes a little cocoon, just like butterflies do. Protected by this cocoon, the flea is impervious to insecticides and can wait many weeks for a host to come near. When the flea senses vibration from footsteps, it pops out, looks around and immediately takes a leap. You can spray all you want, but fleas will continue to emerge from their cocoons for about three weeks.
You can vacuum and spray as much as you like, but expect to see some fleas for 2 weeks or longer following treatment.