So I've made my own nation and I need a few vassals (I have none right now). I captured a castle to offer to any vassals and I always send my envoys to ask them to join. I am at war with the Viegers and I want to recruit a vassal from the Viegers to my side. He hates his king and has a 42 releation with me, but every time I ask he declines because "the lay of the land would put his family and faithful retainers in danger"; thats the only reason he won't join. How do I fix this?

EDIT: Tryed it on a vassal from the Khans (peace treaty) with 40 releation. Said the exact same thing.