You're moving the damage from one target to another before the resistances and vulnerabilities are applied.
Being vulnerable to a damage type means a creature takes extra damage from that damage type. Vulnerability appears in a stat block or power as “Vulnerable x,” where x is the amount of the extra damage. For instance, if a creature has vulnerable 5 fire, it takes 5 extra fire damage whenever it takes that type of damage.
Thus, the target takes extra damage when it takes damage. It must actually take damage to take the extra vulnerability damage. Likewise...
Resistance means a creature takes less damage from a specific damage type. Resistance appears in a stat block or power as “Resist x,” where x is the amount that the damage is reduced, followed by the type of damage that is being resisted. Damage cannot be reduced below 0. For example, a creature that has resist 5 fire takes 5 less fire damage whenever it takes that type of damage.
Resistance reduces damage when it takes damage, not "when damage would be dealt to it."
Whenever an attack you make would damage an ally, you can choose to take the damage instead.
By my reading, if an attack "would" damage an ally, that means it hasn't yet damaged the ally. That is to say, you are interrupting between the "hit" and the "damage" portions of the effect. This means the ally's vulnerabilities or resistances would not yet be factored into the damage.
Whenever you hit with a power that has the radiant keyword, the target gains vulnerability 10 to radiant damage until the end of your next turn.
The fey beast is definitely the target, and definitely getting hit before you step in with Atonement and take the damage yourself, so the vulnerability should be placed on the beast and not transfer over to you with Atonement (Atonement says you take the damage, not become the target).