Please re-read the above. VfE does not "turn the attacker into the defender." It changes all references to "defending player" while satisfying claim to the target of the event (commonly, but not necessarily, the attacker since the person playing VfE can choose anyone in a Melee game) rather than to the original defending player.
We know this because the text on VfE specifically calls out, "That player must satisfy the normal claim effect instead of you as if he or she were the defending player." As a replacement effect, this means that when claim resolves, the target player replaces you as the "defending player." The target player is treated as the defending player when claim is satisfied (i.e., the claim effect resolves), but it does not transform the target player into the defender of the challenge.
Honestly, applying your Calm while VfE has me satisfying claim is the situation alluded to above where there are two people acting as "defending player" at the same time.
Also, since it's a substitution while claim being resolved, instead of a transformation of player identity in general, the target of VfE will not be able to play his own VfE to deflect the substitution back to the original defender. He is only treated as the defending player when claim is actively resolving - which is after interrupts are triggered.
Note also that all of this happens long after the winner/loser of the challenge is determined, so the original defending player (who presumably played VfE) will have already played all of their "after you lose a challenge as the defending player" Reactions.