"usted" is a pronoun used to address someone formally.
What is there in the Serbian text that shows that formality, different from "tú"?
Could you build the line in both ways just to manage to understand the difference, please?
"Nedostaju Vam više nego što mislite." - used to address one person formally
"Nedostaju ti više nego što misliš." - used to address one person informally
"Nedostaju vam više nego što mislite." - used to address more than one person both formally and informally