Can Buddhists make very eccentric vows? Can Buddhists make very eccentric vows, and in what sense might these affect their karma? 

I do believe in karma, in some sense, but am struggling to understand something specific -- and a little strange. Suppose person A vows that, as person B is about to be hurt, they will suffer in their place. The event occurs, but person B, in some ill defined sense, does not suffer as much as they otherwise would. Person A then suffers some grave misfortune (perhaps unconnected).

Is person A at fault for their misfortune, karmically speaking? 

But this is a just so story. Specifically: is there any form of vow, in any tradition, which can lead to suffering for you, but good karma for others?
Not sure if vows alone are sufficient to auto-magically make things happens as one wishes. It's much more likely that intention coupled with concrete actions that will bring about the desired result. So in the case of person A, the event's more likely to happen due to s/he following up with actions that helps person B and maybe at the cost of some suffering to himself. About the concept of collective kamma, it probably is just individual kamma bundled together. An example is the atrocities committed by soldiers during a war campaign. Together they all took part in killing, raping, and plundering their enemies' city. So in some distant future, some catastrophic event happens to a group of people, which on the surface seems like "collective" kamma, but in fact the underlying driving force is still individual kamma manifesting itself.