Spock
In order to observe local custom and further his understanding of Vallo, Spock showed up to the adult prom in part because the sale of his ticket also seemed to be a worthy cause. He does desire to spend time among and be respectful of the community until he either vanishes back to his world or discovers a way to go home on purpose.
That being said, he did not dress up but in fact wandered into prom wearing his typical Starfleet Uniform. If asked, he regretfully did not arrive with a dress uniform and these were the nicest (only) clothing he had. It was not logical to spend the majority of his stipend upon arriving on clothing to be worn once.
Unlike many of the prom attendees who may have had dinner or otherwise arrived fashionably late, Spock was several minutes early, as he is unfamiliar with the concept of fashionably late. He's not a wallflower, precisely. Spock spends much of his time investigating the enchanted effects of the rented space. He is fascinated by the magical décor, as it's his first up close encounter with magic after arriving in Vallo. While not intending to be on theme, one might call him a fish out of water.
He does not eat the food or drink alcohol. He will want to interview anyone that takes the mystery shot to ask what the effect was, what happened, how long, and perhaps most importantly why they took the mystery shot. (Fun? What do you mean taking a mysterious beverage with an unknown randomized magical effect is fun?)
Interaction Ideas With Spock:
"Spock, why are you staring at the walls/magic decor?"
Is anyone going to try and ask this pointy eared dork to dance?
Offering to magic up/Cinderella suitable clothes for the night.
Spock spots you after taking a mystery shot and comes over to interview you.
General conversation topics with Spock...
He will be curious about these promposals.
Do you have prom rituals on your homeworld?
General talk about Vallo and social events and customs beyond prom.
Fully popping the canon puncture bubble with him.
It would be illogical to limit yourself to these ideas, they are merely suggestions.