SOLUTION
I was wrong. Xauthority was not the culprit, Xresources was. I had also merged Xdefaults into Xresources (as Xdefaults is deprecated) and had forgotten about it; that merging was the source of my troubles. (I had a naked variable in Xdefaults intended for urxvt which vim was trying to honor, causing it to crash with invalid parameter attributes
).
My apologies for the confusion.