[RPG] Radiant Vulnerability + Vulnerability All + Pervasive Light

If I'm not mistaken, your Example 2 actually encompasses two different cases, so I'm restating your examples and breaking out my answers accordingly:

Example 1: Creature is Vulnerable 5 Thunder and Vulnerable 2 Lightning. [A single] Attack deals Thunder and Lightning damage.

Example 2a: Creature is affected by [A single effect dealing] Ongoing Fire & Radiant 5, while it is Vulnerable 5 All.

Example 2b: Creature is affected by [two separate effects dealing] Ongoing Fire 5 and Ongoing Radiant 5, while it is Vulnerable 5 All.

Which should result in the following rulings:

Outcome 1: 7 extra damage.
Outcome 2a: 5 extra damage.
Outcome 2b: 10 extra damage.

Assumptions:

• "Vuln [type] [#]" means "whenever the creature suffers a hit of damage type [type], the creature takes [#] extra points of damage."
• Whenever a creature suffers a discrete incident of damage, evaluate that damage in isolation against all the creature's vulnerabilities, resistances, triggering effects, etc. etc.
• If a single damaging effect has two types, all points of damage are considered simultaneously to be of both types; 5 fire&radiant is not 2.5 fire / 2.5 radiant, and thus is not two separate 'hits.'

Conclusions:

• In example 1, the creature suffers a single instance of damage which just happens to deal Thunder and Lightning damage. Evaluate the hit against all the creature's vulnerabilities:
  1. The creature's Vuln 5 Thunder triggers (since the damage is typed Thunder, it meets the criteria for this vulnerability).
  2. The creature's Vuln 2 Lightning triggers (also meets the criteria for this vulnerability).
• In example 2a, the creature suffers a single instance of damage which just happens to count as both Fire and Radiant simultaneously. Therefore the "Vulnerable 5 All" gets triggered once (the typing of the damage [Fire&Radiant] meets the criteria for the vulnerability).
• In example 2b, the creature suffers a separate instance of Fire damage and Radiant damage - two separate "hits" trigger the Vulnerable rules two separate times, exactly as if two separate instances of example 2a had occurred.

NO

You're moving the damage from one target to another before the resistances and vulnerabilities are applied.

Vulnerability^ddi

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^ddi

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."

Nusemnee's Atonement^ddi

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.

Morninglord^ddi

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).