When Van Helsing, Harker, Godalming & Morris hide in the cemetary outside of her crypt, they catch Lucy returning at dawn. She 'turns to mist' in order to pass through the crack beneath the door in order to reenter her tomb, in full view of the men, who then find her in her coffin--- which was sealed with lead. So one must assume that she also used the ability in order to leave and return to it as well.
But why was Dracula weakened? If I recall correctly, it was not because he had been done any physical harm, but because his resting places had been violated. He was vulnerable in the sense that his places of rest were no longer secure, his presence was known, and he was being actively pursued by men with greater resources and the strength of numbers.