Well... not quite. The selection effect makes the survival number basically impossible to calculate, but regularly surviving risky scenarios seems like it would provide a bit better odds for the influence of moxie than 249:200.

At some point, if the Vulcan is smart enough, I suspect the calculation would begin to hinge more on plot twists and the odds that the story is nearing its end, as the hypothesis that they are wearing Plot Armor rises up to the forefront.

I'd also suspect that the Vulcan would realize quickly that as his prediction for the probability of success approaches 1, the odds of a sudden plot reversal that plunges them all in deep poo also approaches 1. And then the Vulcan would immediately adjust to always spouting off some random high-odds-against-us number all the time just to make sure they'd always succeed heroically.

Ow, this is starting to sound very newcomblike.