Sam, Samwise The Brave can prevent a burden from being added using Vision From Afar although the FP player adds the burden, not the shadow card. That can't be right, is it?
I wish the rules were any clearer, but here is something that remotely might explain how the cards work:
The source of an effect is the card on which that effect is printed. Even though a card like an event may require a minion to exert to pay its cost, the source of that effect is the event card and not the minion.
So it's not the FP player adding burden, but the shadow card, it's just the FP player making a choice to add burden or not.
Anyways, there is a different rule creep in this case:
If something happens to prevent one effect which in turn would have prevented a second effect, the second effect is performed.
So, in case of this interaction, here is what IMO should happen:
Shadow player plays
Vision From Afar, FP chooses to add a burden to prevent the effect. FP player has an option to prevent the burden with Sam, as the source of the burden is a shadow card (it's printed on it). But if the FP player chooses to prevent the burden the original
Vision From Afar effect (assigning minion to Ring-bearer) will still happen.
I hope that's correct.