Can Buddhists make very eccentric vows?
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.