“And you extrapolate from that, that Marshall must have also found Vattel’s supposed opinion on citizenship relevant.”
Since we are extrapolating how about we extrapolate this:
“The constitution gives to the president, in general terms, “the power to grant reprieves and pardons for offences against the United States.”
“As this power had been exercised from time immemorial by the executive of that nation whose language is our language, and to whose judicial institutions ours bear a close resemblance; we adopt their principles respecting the operation and effect of a pardon, and look into their books for the rules prescribing the manner in which it is to be used by the person who would avail himself of it.” Chief Justice Marshall, in United States v. Wilson 1833
All this says is most of our Legal system mostly bears a close resemblance to that of England. It says nothing of the particular instances in which the US legal Doctrine deviates greatly from British Law. Among these areas, citizenship vs servitude is one of the most pronounced. We fought two wars in order to establish the distinction between citizenship and subjectude.
In a Monarchy, it is very much in the interest of a King to force allegiance on anyone upon whom you can throw the yoke. They are, after all, your servants. Why would you want to exclude anyone? An expansive definition of whom can be claimed as a "subject" suits the best interest of the Crown.
A Free Republic operates under a different premise. You are not born into the service of the King, you are free and independent while being able to chose to whom you wish to give your allegiance. It does not use any and all pretexts to throw a perpetual commitment upon you.