There's plenty of danger in an imperfectly thought out vow, even if you add a clause that someone can release you from it. Having someone who could release you from your vow isn't much help if you're already dead due to having been unable to act in self defense, for instance. Supposing you have to go down to the equivalent of a local police station to get released from a Vow, I would suggest that this probably retains most of the problems of being unable to break the vows at all.

You are right, Harry did not add enough layers of precautions. As such, he is in a position for it to screw him over. A truly well thought out Vow would have several escape clauses like this, to different people, and also a clause temporarily suspending the Vow while you go to have it removed if you truly believe that the situation warrants its removal.

Even that might not be enough, but remember: Harry thought his oath out in less than a minute. I thought out my additions in not much more. I imagine that if someone smart brainstormed this for a couple of hours, they would figure out even more elaborate and foolproof mechanisms. And once it gets going, there will be people who spend their whole careers on the question, refining the answer even further.

If you think it doesn't matter which, I have to suspect that you're not thinking very hard of the implications of the MoR method.

I think we have a very large difference of opinions here. Remember that in the real world, most societies with slaves lasted quite a long while. Add onto that several additional factors that are greatly to the advantage of the masters and not to the advantage of the slaves, and you see why I think there is not much of a difference.

• Numbers. In the example you gave, of the Helot system, please not that there were seven times as many helots as non-helots. That's right, seven times as many. In this system, you would not need even nearly a one-to one relationship. I would guess that there would be probably no more then one slave needed for every five or six people. This reduces their ability to revolt by so much that it is nearly impossible to compare it.

• Magic. I would envision the slaves not having access to wands except when binding a Vow, giving access to wands to only a small fraction of the slave population at a time. Add on to that the fact the the slaves would not get much magical training, and that they have their magic reserves permanently depleted, and you will see that the masters have another insurmountable advantage.

• The Imperius Curse. During the time when the slaves do have a wand, they would be under the Imperius Curse. This would not need more than one slave to be controlled by someone's Imperius Curse at any one point. And remember, when you say that not everyone can be controlled by this curse: In canon, it was a huge surprise that Harry managed to throw it off so easily, the kind of thing you can only get if you are the hero of the story. Most people resistant to it are only that way due to a large amount of willpower built up over the years. And you can just execute people like that.

• Unbreakable Vows. I don't think these are needed, as the other factors cover the possibility of a revolt very well, but if needed, some slaves could be forced into an unbreakable vow not to rebel.

far from other countries seeing and wanting to copy this system, they're liable to see it as either an exploitable weakness or a human rights violation, in which case this society could be facing trade sanctions, embargoes, or even war.

Remember that the concept of "human rights violations" are something very new, and they did not stop basically every nation that ever existed from keeping some sort sort of slaves. Remember that for thousands of years, there where, sadly, a lot of slaveholding and other human rights violations. And in no case did this result in the moral outrage from the surrounding countries, and certainly not to the point where they declared war or started trade embargos. (By the way, just thinking about it makes me very much appreciate the moral progress humanity has made in the last few hundred years. It truly is incredible if you think about it.)

And of course, you still have the issue of how society undergoes the steps to reach this point.

We have discussed this a lot, and have provided a lot of explanation about how I do not really consider this an issue, and others have chimed in, adding their views on the subject. After all that, I believe we have provided many different angles of the explanation of why this is. If there is a specific part of it that you object to or want clarified, I am happy to discuss it further, but other than that I am not sure that there is more to discuss.