what's the quickest way to get the vaginal skill along with other skills revolving around sadism?
Sadism. Just beat prisoners up or use sadism-oriented skills (footjobs, vaginal sex (skill), anal sex). You can try to build it on 2 level, there Nerds that loves footjobs. And don't forget: use cock kick a lot. And keep desires under control: external ejaculations buffs Karryn sadism.
About vaginal sex. First thing, you need enough pussy and cock desire for that (you can buy accessories that lowers desire for it). Second, you need to engage in vaginal sex until you unlock passive for skill. There are the guards, thugs, lizardmen, who initiate a vaginal sex with Karryn.