The log hinge-vice fail: thought about editing that out, but chose to leave it in because it could still work, and it's kind of an interesting idea.
So what went wrong?
First of all, the log was too old. If you look at the top of it, there's a discoloration that sort of resembles water-staining. That's the start of the tree's decomposition, and it's the first sign that the wood is becoming punky. I had remembered the tree being fresh, but I was mistaken. The hinge was also too short (and misaligned). It was another case of worrying more about camera shots than the project. Seasonal dryness was also a likely contributor to weakness.
Another possible approach would be to make the log vice sideways. Because vertical wood is much more flexible, rip cuts made on either end of the log could produce a much tougher hinge. There would be two complications here: 1) horizontal logs like to roll, so a flat spot would have to be cut, or else sawhorse-style legs could be added; 2) logs have a limited diameter which will provide limited leverage, so the working range of the vice will have a much smaller tolerance.