I don't know if we'll have any private vendors besides our photographer (and the resort) but our photographer does require full payment 3 weeks in advance.
It does feel like a bit of a risk, but you would hope that the risk to them of being "outed" in reviews would keep them honest. As you note, even one bad review can damage their reputation. If they failed to show (and provide a replacement) they'd see the affects pretty quickly.
You've done your research so if your gut feels good about it, I think you should go for it. Our guts rarely lead us wrong