>> “So you are reduced now to making the argument that the founders were more familiar with Vattel than they were with English law?” <<
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They were familiar with both, but they studied almost exclusively in the French language during that period. Things expressed in the English language were considered crude and erudite.
The system of law that the American founding fathers were most familiar with was English law - not the writings of Vattel - which never were codified into law in any nation.
John Adams, a founding father and a lawyer by vocation, was sent to represent us in France - and was considered an unlikely choice because he did not at the time speak French.
He became fluent only after living in France. So it seem he, as just one example, did not study almost exclusively in French.