Do you think Vincent would have let the jazz man live had he guessed right?

Nope.

He was dead eitherway. Vincent was a professional.

If he did, then maybe Vincent would have given him very specific instructions on getting on a plane to leave far away. I don't remember if Vincent actually made any straight and firm promises that he broke in the film.

I think if he broke that promise, then it was because he didn't trust the jazz guy to do as he said, which to be fair he already had trust issues with Max several times.

Not a chance. It didn’t matter whether he got it right or wrong (or if Vincent even took a liking to the Jazz cat), Vincent was gonna fulfill his obligation regardless. Man had a reputation to uphold.

It would have surprised the hell out of Vincent if he did, but he would have murked him anyway.