I think I have been reasonably well informed for some time on this. I know, for example, that most of the female reproductive system is internal and that the vulva refers to those parts which are visible externally. I know about the labia majora and labia minora, that the clitoris is at the top of the vulva, the urethral opening next and the vagina lowest. Also that, while the opening is visible from the vulva, that most of the vagina, the tube into which you insert your penis, is internal with the G-spot on the front wall and that the hymen, when intact, covers the entrance to the vagina.
In order to bring the case referred to within the rules, if we imagine instead that we were considering whether a sexual assault had taken place on an adult, virgin woman I would very suprised if the state of the hymen made a material difference. The definition of sexual assult would have to be rather narrow to require that the vagina had been penetrated. It seems to me that if a man were to put his penis between the lips of the vulva without consent, even if he did not get as far as going into the vagina, the woman concerned would feel violated and I would very much hope the law would see this as a sexual assault.