None of the underlying mechanics are defined, particularly possession-by-diablerie
The books don't really even bother to define what happens during diablerie, only the mechanical process for committing it. They offer no guidance whatsoever on how a vampire might overwhelm a would-be diablerist, or how the diablerist might overwhelm their victim. The relevant rules section reads
A few rumors speak of diablerists displaying certain
mannerisms of their late victims, particularly if the victims
were of great psychic fortitude (Willpower 10) and
of much stronger Blood than their murderers. If this is
true, and the soul of a particularly mighty undead can
manifest in the body of its killer, the implications are
frightening, particularly in light of the Jyhad. (V20 Core Rulebook, page 295)
That's it! Though in contrast to many vagaries in V:tM, we at least have canonical examples of the possession happening narratively. So there is no information on how or when vampires overwhelm each other during a diablerie attempt, or even when such a contest does or does not happen, let alone what might happen during a specific type of diablerie instance.
There is no information on when possession-by-diablerie definitely does or does not occur, nor in how the process plays out in cases where it does occur.
I'm not sure it is necessarily any of those options. Each might be possible under different conditions, though a normal Embrace seems most in line with published information
My basic reasoning is this:
- It is possible to drink from a vampire, even to Final Death,
without attempting diablerie at all
- Diablerie is intentional and requires considerable effort at the moment it is attempted
- The opportunity for a would-be victim of diablerie to possess their
attacker is itself a product of the struggle diablerie entails
- Humans (and, I believe, all non-vampires) cannot be diablerized, nor
can they engage in diablerie
The first and second points suggest that your first possibility is most likely: the single drop of blood (or whatever volume is used) fuels the Embrace, after which the vitae is the new vampire's own blood and no longer a target for diablerie. You may or may not have a case in which there is a large enough volume of blood comprising the debitum to fuel the embrace and then have some left over for diablerie.
Diablerie is not something which a vampire can engage in by accident (as far as I'm aware, if there are instances of incidental diablerie I would appreciate a comment with a citation!). I don't see a point at which the deceased mortal can be "drinking" the Debitum as a vampire, and initiate the struggle to win or lose.
The third and fourth points suggest that, per officially published mechanics and plot events, the vampire from whom the Debitum was created won't have the opportunity to overwhelm the mortal and take over their body (at least via diablerie). You can't just create a diablerie event, with yourself as the victim, as a result of being at the cusp of Final Death. Other methods of enduring death certainly exist (cf. Tremere and Goratrix), but not via diablerie.
You don't get a grace period to attempt the act, either. If you drink all of a vampire's blood and send them to their Final Death, they die. Their blood is yours, but not their soul, and it's too late to get the soul from the blood-- the blood is yours now, and the soul is either gone or not accessible via diablerie any more.
Finally, mortals can't initiate diablerie, or (I believe) participate in it at all. So while a vampire's soul might still exist in a Debitum, the mortal is still a mortal and can't be diablerized. When the mortal has been Embraced and is a valid target for diablerie, the act of drinking is over and the chance for diablerie has passed.
Antediluvians are weird and shatter the rules
Antediluvians fundamentally break all story rules. They are consistently described as being powerful beyond imagining, and the books frequently indicate that they break rules which otherwise have no known exceptions. This makes comparisons to published information hard, as most of the instances of diablerie which don't follow the standard mechanics from the core rulebook involve Antediluvian victims. We can't really tell if diablerie could just work that way, generally, or if it's the Antediluvians' amazing powers and superhuman capacities that transcend what are inescapable rules for everyone else.
I recall only one instance at all similar to this in the published books
And it's kind of the reverse situation. I'm having trouble finding the specific reference (I will update once I've had time to dig through my collection), but I believe that in one of the Giovanni clanbooks there is some discussion of Augustus Giovanni's diablerie of Cappadocius being incomplete due to a small amount of the latter's blood being preserved in the True Vessel. This already breaks the rules we know, because Augustus did gain a great deal of power from the act even though it was, apparently, incomplete. Augustus has also (reportedly) been desperate to obtain the True Vessel and complete his task.
These do suggest that vitae can be stored externally and possibly longer than the vampire it came from survives, does (or can) have a portion of the soul of that vampire in it, and that that portion of the soul can be used as part of committing diablerie.
But too much is unclear to give good guidance in any general case. Is that legend true at all, in-game? Is it related to Antediluvians' game-breaking qualities? Is it related to Cappadocius' knowledge of and relationship with death and the afterlife, with or without Mortis? Is it because Augustus already mostly diablerized Cappadocius? Who knows?
