So, I don't so much have an organized theory as a few points I would like to make.
1. For the purpose of full disclosure, I admit that I am most in favour of the something-in-the-vault theory, though I am also slightly suspicious of it.
2. I don't know if the Harry Potter comparison is entirely fair. The power of love group of theories seem to centre around the idea that Steelheart was made vulnerable because Dave's father was attempting to protect him-Steelheart from Deathpoint, that that protective intention was what allowed Steelheart to be harmed. (In HP, Harry was protected from Voldemort's curse by the fact that his mother died to save him from said curse and curser... Which in itself paints kind of a dark picture, as that was apparently the first time someone sacrificed themself for another person during Lord V's whole reign of terror. Real nice, wizards. But I digress.) Anyway, I think there is some merit to the intent theories, because A) all epics are evil how better to describe evil than to say it is a lack of compassion, or love, for other people? C) Since he can't understand compassion/love/protctiveness, the psychic armour or whatever that protects Steelpoint is rendered useless by it.
3. About vault theories: I think it is relevant that Deathpoint, a weaker Epic, can't effect bones or metal, which are comparatively more solid, more dense, than anything else on or in a person's body. (I'm kind of curious to know what happens to a person's glasses.) Gold, which could reasonably be stored in a bank vault, is really, really dense - perhaps dense enough to interfere with Steelheart?
4. I think that if Steelheart were vulnerable to accidental damage, he would be in serious trouble the the first time he went on a building-wrecking rampage and knocked out a load bearing wall. I'm no epic, but probably a solid ninety-five percent of my total lifetime injuries have been the results of accidents... So, seems like that would hardly be a good path to invulnerability. :-) (And the Steelheart menace was ended with the Epic carelessly crossed the street without looking both ways....)
Anyway, those are my thoughts, can't wait to hear how you argue with then!