Well, I'm concerned with usage and not etymology. To me, vos vs. usted is still a T-V distinction. Of note, it's called a T-V distinction regardless of whether the V-form is etymologically a second-person plural as in English and French, a third-person pronoun as in Italian and German, or a separate construction as in Spanish and Portuguese.
...In contrast, the T-V distinction is exclusive to pronouns, and is not eroding due to any sound shift. There has to be some separate sociological explanation for why English got rid of the T-form.
-Fair enough, the only usage I've seen of the terminology in literature (not that much) deals explicitly with the PIE roots of those pronouns more than actual use.
-If we're going the sociological route for an explanation, which is certainly possible, I'd wonder if the Renaissance left any sort of mark there. I'm not sure how it would in any explicit way, but it seems to sync up with the 16th. Century slide away from "thou-ing" in living speech, more as an inference than anything.