Since Spock didn't change into a "Vulcan tux" ( or a dress uniform) we can assume that T'Pring is dressed in Vulcan casual.
I see no reason to make such an assumption, especially given T'Pau's highly formal dress.
She's wearing what looks like a black pants suit/turtleneck combo (standard alien fair in TOS) with a silver skirt and a cape of gossamer. Is that highly formal on Vulcan????
As I said, the TOS Vulcans didnt go for robes. That seems to be an invention of the movies when the Vulcans went from scientists to mystics.