Yeah, I just came across that, too. Makes it less useful against Tanarukk, but potentially more useful against trolls. (I keep finding conflicting arguments and statements from WotC; one Customer Service answer said vile damage would lessen a troll's HP's, and not convert to nonlethal... but that just flat out ignores the fact that if its not fire or acid, it's NEVER lethal damage to a troll...) But, a fireball affected by Violate Spell would be bad news for a troll, as it would be fire damage, and it would be vile. Not good for the regenerating buggers...
Also, which vestige was it that granted the vile damage? Because it doesn't appear to be Astaroth (which is from the Cityscape Web Enhancement, and grants a stinky breath weapon, and a bonus crafting feat, but no vile damage ability that I can see).
Edit: nevermind, it appears there are two Astaroth vestiges (why in God's name would they do that...?)VILE DAMAGE
Vile damage, like regular damage, results in the loss of hit points or ability score points. Unlike regular damage, vile damage can only be healed by magic cast within the area of a consecrate or hallow spell. Vile damage represents such an evil violation to a character’s body or soul that only in a holy place can healing magic repair the damage.
Man, could they be any more vague and non-specific?
Blackflame: Your spells, spell-like abilities, and magic item abilities with the fire descriptor deal half fire damage and half vile damage. You can cast fireball, as per the spell, three times per day (caster level equals your effective binder level).
Despite the wording seeming misleading, it's basically saying exactly the same thing as the first description of vile damage in the Violate Spell feat ("half the damage dealt is vile damage").
So, with the clarity provided in the example within Violate Spell, what it really seems they were trying to convey is that vile damage is a descriptor, that could be added and coexist with other descriptors [fire, electricity, healing, whatever]. So, any fire spell affected by Blackflame would still deal 100% the same type of damage, but half the damage would also carry, what is in effect, the [vile] descriptor.
So, most of the spells where this comes into play will screw a troll hardcore, because it will be both fire and vile, thus unquestioningly reduce its hit points.
On something like Fiendish Trolls, they will resist some amount of it, but whatever gets through, half inflicted will be vile damage.
Now the only pertinent question seems to be a Blackflame-effected Flame Strike. Is it straight-up half fire and half vile? Or is it 1/4 [fire], 1/4 [fire, vile], 1/4 [divine], 1/4 [divine, vile]?
Also, at some point I may have to figure out how vile damage will interact with Regeneration if it is of a type not overcome by fire. Say something like a War Troll, with Regen 9 (Acid). A [fire, vile] spell effect is not acid, so would not deal normal damage. But I'll have to figure out between the following two:
1) The vile damage is converted to vile nonlethal damage, but then won't regenerate.
2) The vile damage is converted to nonlethal damage, and is regenerated as normal.
I'll figure that out later, though. It won't come up for a long while.