In context, "screw" is most certainly a vulgarity in the sense that a medieval noble would mean.
Just because you didn't use a much more blatant and (in modern times) taboo swear word doesn't mean that what you said was not vulgar.
Try to imagine a medieval noble, full of the notion that status is everything and erudite and elevated language is a true mark of birth, saying anything like "screw you" to another noble—especially in a letter, as opposed to face-to-face in the heat of an argument.
They would be much more likely to say something along the lines of, "If that is your position, then I can but think that you are among those benighted folk who plight their troth with sheep, for it bespeaks a similar level of intellectual and moral fortitude."
As for whether the other party is also in the wrong...sure, he probably is. And he might very well have also had an etiquette issue opened for him, and lost honour because of it. Unless you're the one who opened the issue, or were asked to judge it, there's no way you'd ever know.
