How do I transfer ducal vassals to a viceroy?

How do I transfer ducal vassals to a viceroy? - Person Holding Debit Card

I would like to transfer dukes vassalage to a Viceroy, specifically I would like Duke of Leon to be a vassal of Viceroy of Castille. But this option does not seem to come up. I thought you could transfer any vassal to another vassal so long as they were in the right places in the hierarchy. What are the rules exactly?

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Crusader Kings II Tutorial: Managing Vassals and the Importance of the De Jure Chain.

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