Alzrius said:
It is an effect. Likewise, (this is exactly where I said this'd go) it "improves" the strike by adding an additional effect to the damage dealt. It goes from just being damage to damage plus energy drain.
No it doesn't. It is an effect that targets the target of the unarmed strike which is triggered by the hitting with a natural weapon. The language of the Unarmed Strike ability is so that monks can use either
Magic Weapon or
Magic Fang.
Alzrius said:
Transforming the unarmed strike into a natural weapon is exactly what it does. A monk's unarmed strike class feature explicitly says that it is to be treated as a natural weapon for all spells and effects. Energy drain is an effect that uses the vehicle of natural weapons. Ergo, it does use that.
No. It says it can be treated as a natural weapon or a manufactured weapon for spells and effects that improve one or the other. In no other way is it to be treated as a natural weapon. If a spell negatively affects a target's natural weapons,
it does not work on a monk's unarmed strike. The unarmed strike is not a natural weapon. It can be enhanced
as if it were one, that's it.
Alzrius said:
The list is not, nor is it meant to be, comprehensive. Likewise, even if it was, this is exactly why the monk's Unarmed Strike class feature explicitly calls out its use as a natural weapon.
The list lists the most common. If the unarmed strike was intended to be a natural weapon, the simplest thing would be to call it a slam. The class feature explicitly calls out its ability to be enhanced by either
Magic Weapon or
Magic Fang (Although if you play eberron, a question of whether or not Artificer infusions work on non-construct monks

)
Alzrius said:
Sorry, it just doesn't work. The language is very precise and does not allow for it. By contrast, if the Energy Drain was an effect that charged a natural weapon with negative energy, then it would work.
Instead, it inflicts 2 negative levels on a creature struck by the Vampire's natural weapons and Unarmed Strikes are not natural weapons. This may be intuitively/conceptual wrong, but as far as the rules language is concerned, it is correct.
As a side note, natural weapons are not monk weapons and cannot flurry with natural weapons.
Also note:
SRD said:
Creatures do not receive additional attacks from a high base attack bonus when using natural weapons.
If a monk's unarmed strike
was a natural weapon, he'd get one attack per round.