1. AFAIK vassal_decision is a decision that affects either a vassal, a courtier, or someone you are hosting(you host prisoners and councilors, though councilors must also be a vassal or courtier to be a councilor.)Few questions:
1.What is vassal_decision?
2.How to make decision trigger an event that targets a random prisoner?
3.Why are there no guides for decision modding ANYWHERE?
2. This is just a guess, but most targets are pretty blunt, so it's probably random_prisoner.
3. No clue, I wish there were some. The wiki does have lists of some scopes and commands, though; so that should fulfill most of your needs.