Particle_Man
Explorer
To clarify: Option C is that it deals the bonus hellfire damage, but the energy type is [Acid]?
As written, I see it functioning thusly:
1. Hellfire Blast changes your Eldritch Blast to a Hellfire Blast. This is not an essence, but a change to the actual Eldritch Blast ability. You deal Elditch Blast damage + bonus Hellfire Blast damage. The damage type is [Hellfire]
2. The Vitrolic Blast Essence is added to this Hellfire Blast. The [Hellfire] damage type is changed to [Acid], and there is additional acid damage dealt in the following rounds.
I would argue that if the Hellfire is changed to Acid, then it is no longer hellfire and no longer does the bonus damage. If the Acid is changed to Hellfire, then it is no longer acid and no longer auto-penetrates SR.
As slightly more evidence I note that, despite gaining "new invocations known, increased damage with eldritch blast, and an increase in invoker level as if you had also gained a level in the warlock class", the hellfire warlock "offers great power at the expense of versatility". Since the loss of Con does not by itself mean a loss of versatility, my assumption is that the loss of versatility means you can't use an essence at the same time as you use hellfire blast. Indeed, the naming convention (changing Eldritch to Vitriolic, Brimstome, etc.) of essences applied to eldritch blast seems to be followed by the changing of the name of Eldritch Blast to Hellfire Blast.
I suppose if one wants to be really picky one can say that the text that states "you can change your eldritch blast into a hellfire blast" does not state that "you can change your vitriolic blast into an eldritch blast" and in Complete Arcane essences are said to apply to eldritch blasts, not to hellfire blasts. I interpret hellfire as having the functional equivalent of eldritch essence status. I don't think the rules say my interpretation of the rules is incorrect although I can see there is room for alternate interpretations.
I don't believe there are other cases of something "mixing types" like a fireball that also deals acid damage. The closest I found was the feat "energy admixture" but even there the fireball does not do (for example) 6d6 fire/acid, but 6d6 fire and 6d6 acid.
Under its own description hellfire is described as an energy (p. 120, knowledge arcana), so I don't see it being compatible with other forms of energy. Otherwise, hellfire would logically also be fire and deal fire damage, but it isn't and doesn't, according to the rules (fire resistance/immunity doesn't work against it, it works vs. objects, etc.)
However, all of this is beside the point, as far as the OP is concerned. I would suggest the OP check with your DM to see if you are allowed to have a vitriolic hellfire blast. If so, go for it. If not, apply some means of overcoming SR (via feats, items, etc.) when you use a hellfire blast.