vuestras mercedes (or "vuesas mercedes"), five or six centuries ago
ustedes, nowadays
Basically, "vos sois" is addressing one sole person by using plural, a polite form that is today common in other languages. "vuestra merced es" is addressing a person in third person, a polite form that is today common in other languages and Spanish (usted, ustedes).
Today, both expressions doesn't sound anything but dated, and the degree of formality is just in the ear of the "beholder".
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