Ask your Storyteller, they are always right
I remember that some systems for Diablerie were suggested in some sourcebooks. However, I remember them to be mere suggestions. For example, some sentences from page 355 from DAV20:
Moreover, drinking the vitae of elder vampires can
induce a temporary increase in the diablerist’s Discipline
levels by one, two, or even more dots, as the potent
blood augments the predator’s own mystic arts. If the
elder vampire was several Generations removed from
the diablerist’s own Generation, the effects can seem
miraculous, even if they are short-lived. These increased
powers last for a single scene, unless the Storyteller
One can be tempted to read that as "you certainly get your Disciplines temporarily increased", but notice the words "can" and "unless the Storyteller decides otherwise", the uncertainty about the number of dots, and the following line in the previous paragraph (about the Generation decrease):
If the victim was of far greater power (five or more
Generations lower) than the diablerist, the Storyteller
may rule that the predator lowers her Generation by
more than one step.
While doing further research on the subject, I have stumbled upon the Diablerie system of "The Black Hand: A Guide to the Tal'Mahe'Ra", a supplement for V20 edition. This book has some very detailed mechanics of Diablerie on pages 180-181...
...prefaced with the following paragraph:
The following optional advanced play systems and
story ideas expand upon the standard rules presented
for diablerie (pp. 293-295 of V20) and Golconda (pp.
299-301 of V20). These tools provide step-by-step rewards
and perils to supplement an existing narrative through
guided effects. Both offer in-depth mechanical accuracy
that may not be suitable for every table, and Storytellers
should not feel obligated to incorporate these systems if
they prove disruptive to their chronicle. Be forewarned,
the devil is in the details, and minutiae can just as often
enhance a game as collapse it.
All of the Storytellers that I have encountered have used their own systems for Diablerie, so don't even bother looking up that stuff in the books. Most of those systems make this process somewhat unpredictable.
I will not list them because...
The Storyteller will likely not answer your question
Most characters in the World of Darkness don't know much about the Amaranth. To be fair, many members of the Camarilla don't even know that it's possible.
It is unlikely that your character knows a lot on the subject, hence there is no reason to provide you, the player, with this information.
Can your Storyteller nerf you if needed?
There are different playstyle doctrines. Some of them say "Damn yes!", some say "No, under no circumstances". If you think the latter and your Storyteller thinks the former, you are not on the same page and that is a problem bigger than your original question. Different playstyles usually don't mesh very well, either of you has to adapt, better both of you.
My personal belief is that nerfing players is OK, but it should be discussed with the player in question. Your opinion might be different, so can be the opinion of your Storyteller.
Is Vicissitude a good addition to every character?
Definitely no. Having or, sometimes, not having a Discipline is a plot device that shapes the character. Many Storytellers I know believe that Vicissitude shapes one's personality, not just the body, so gaining it is actually a severe plot twist.
Maybe your Storyteller didn't understand that giving you this power changes the game. Maybe they did understand it but didn't find an in-game way to deprive you of this power.
What should you do now?
Talk to your Storyteller outside of the game. Better do it in-person. Discuss the issue presenting all of your concerns. Tell why you want this power and the related plot twist, and how it should affect your character. If you want to upgrade this Discipline, discuss the powers that are going to suit the story and those that are not. For example, do you plan to assume the Zulo shape, which gives you +3 or +4 (I don't remember the Revised version) to all of your Physical Attributes?
If you make an agreement, for example, "OK, you can keep this power, but you don't advance it further than the first dot for plot reasons", stick to it.
Be prepared for a "No". You can certainly ask for the basis, but in WoD it can be as simple as "Because I am the Storyteller and I don't want it in my Story. End of discussion."
What should you do if you are not allowed to have this power and you are really pissed off by that fact?
Maybe find another Storyteller. Maybe your groupmates will even join you. Maybe not.
But if there is a severe playstyle mismatch, your best bet is usually finding someone with whom it is actually fun to play (for you).
Note that it doesn't mean that you cannot stay friends. You can still spend time together -- just not around the gaming table. Maybe you can even play another tabletop RPG that is not WoD-related, as many people behave very differently in different games.
Anyway, good luck on your journey, and have fun.