Thank you for your reply. I'm far too curious about what it would take to satisfy a succubus to not try the second option. So far I can tell you that Endurance at 25 is not enough. I shall keep testing and reply to this with the necessary stats.
I can say that it is possible to circumvent this entirely by defeating Vicoria in battle (lust or hp, the end result is the same). The barrier will fade and you can speak to Ste who will rush to Vicoria, say it was consensual and that you didn't have to resort to violence. You will get a "Ste likes you less" message, however. Nevertheless, he will leave and give you hints to find him.