Sciencepone of Science!

@FilthyRichFan25

She wanted to tell the guy the truth from the start, so when Vapor came out and said it she thought it was a good thing. She even keeps smiling for a while till things really start going bad.

@FilthyRichFan25

Looks like a nervous "I'm not sure how this will turn out" smile to me.

It's more like failed pokerface, than a sociopath's smile

See:I think it's some combination. She's not sure how things will turn out, is very nervous, but is also glad the truth is coming out, but mostly that looks like a nervous grin, kinda like the nervous grin Pinkie made in "The One Where Pinkie Pie Knows".It's got the stressful creases at the corners, the gums visible in the sides, and the upper and lower teeth being slightly mis-aligned, and the slight raise at the bottom of the eyes, so it's a stress smile.So, basically, a Twilight's not a jerk.