I posted an image of the letter - it is not great, blurry, but it’s the best I could do at the moment.
Transcribed:
(skipping the salutation)
As Mr. Vaughan was favored last Spring at Aranjues with a passport from his Excellency the Count De Florida Blanca to go and reside at Toledo, I omitted to enumerate in my last, the circumstances requested in your letter.
This gentleman’s father is an Englishman-his mother is an american. He himself was born, I think, in England. He means to become a Citizen of, & to settle in, one of the United States-he is by Profession a merchant. (skipping the resume of Vaughan)
He is now desirous of going to Cadiz, that he may during the Winter form proper commercial Connections there, and in the Spring embark for North America.
He has offered to take an oath of Allegiance to the United States, before me. I advised him to postpone it”till his arrival there, as well because I thought it most proper in itself, as because I did not conceive myself authorized to administer it.
This Sir is a short but very candid account of what I know of this Gentleman ,
*etc - the rest is commenting on Vaughan’s character being good.
I don’t think it is as significant as you think. Vaugan was born in England before July 4th, 1776. His mother might be American now but she could not have been when he was born (unless he is 4 years old). He is without question a British Subject.
You can find the letter here. I can provide a screen shot if you wish.