Note that the thread title pertains to PvP—the two mentioned classes are not buffed in PvE since NPCs do not use %-based mitigation as part of their defenses (AFAIK; please correct me if I'm wrong).

So a lot of PvP players have reported taking noticeably more damage since Wrathstone landed, and this new bug/feature appears to be one part of the puzzle—vulnerability debuffs are now calculated additively with your %-based mitigation by default.

Background info:
Prior to update 21, vulnerabilities (debuffs that increase your damage taken, eg. minor vulnerability from the Shock elemental status, etc. etc.) were calculated multiplicatively in the damage equation, as per this excerpt from the Damage Mitigation: Explanation thread by @paulsimonps.
However, since Wrathstone's launch, vulnerabilities have become additive with your %-based mitigation, via this adjusted partial* formula:

Damage taken = (%-based mitigation + Vulnerabilities)

Where "%-based mitigation" represents the product of all the complements of your %-mitigation sources eg. (1-x)(1-y)(1-z)

* resistances, block, & execute multipliers (on the attacker) are excluded because they are all multiplicative with the above sub-expression, and thus do not influence the proportional damage increase

The consequences of this aren't immediately obvious, so here's an example calculation, comparing pre vs. post-update 21, given the following target stats (quite standard for a CP810 toon):

• Stage 4 vamp (+25% fire damage vulnerability)
• 66 pts Ironclad (22 %-based mitigation)
• 37 pts Elemental Defender (9 %-based mitigation)
• Minor protection (8 %-based mitigation)

From the above workings we can see that fire damage dealt to our hypothetical target, post-Wrathstone, increased by a whopping 10.6%, without any changes whatsoever to the stats of attacker or target. The magnitude of this increase correlates with the amount of %-based mitigation the target stacks, as the Vulnerabilities term (a flat value) grows larger in proportion, relative to the shrinking value of the %-damage taken by the target.

As referenced in the title, this change primarily benefits two classes:

1. MagDKs: Nearly all their damage output applies to the +10% fire vulnerability debuff from Engulfing Flames—which is boosted by this mechanic—AND this stacks with the Vampire fire vulnerability
2. Nightblades: The "20% increased damage taken" debuff of Incap/Soulharvest is classified as a vulnerability, thus all follow-up damage is boosted by this change

Minor Vulnerability also benefits, to a lesser extent due to its smaller 8% value. Arguably favours Magsorcs—though any class can readily access it via shock glyphs or poisons. Also note that Dawnbreaker and other Fighters Guild skills do not benefit, as the "Skilled Tracker" passive is factored as a multiplicative bonus to the attacker, not a vulnerability on the target.