Rumours speak of a sacred few who have escaped the Curse of Caine and
become mortal again. Such stories of Redemption have existed for
centuries, as have tales of eternal Damnation. If it seems outrageous that a vampire can permanently change his state of being through magical ritual, vampiric history offers many precedents.
e.g. Golconda, the Crimson Pool, the Tremere, Apotheosis and Amaranth.
The Red Sign
There's actually an entire book that deals with this called The Red Sign, much like there is for everything else in oWoD, from which all the quotes are taken and naturally it's the Mages who would be the ones to accomplish such a feat. The method - the ritual of the Red Sign
The Red Sign is at the heart of another legend of life and death. It is
the culminating movement of a communal ritual, one that can return the
undead to life. Like other legends of those who have overcome the
horrors of vampirism - whether by taking the Beast, walking in
sunlight for one blessed day, or simply escaping the madness and
persecution of vampiric society - such achievements are mythical,
unsubstantiated and ephemeral. No one has ever proven that the Red
Sign has been used to redeem the damned soul of a Vampire... but it is
only a matter of time.
There is a LOT of detail regarding the process of discovering the ritual, the tome that holds the knowledge of it, various tales and stories as well as possible ways of incorporating it into your own adventures. Rules-wise it is a difficulty 11 ritual requiring 10 successes as standard, although it is fundamentally up to the storyteller.
The book suggests that it might be easier (difficulty 11, but with a -3 mod available and only 5 successes) or even functionally impossible (30 successes or difficulty 15+) to succeed at the ritual depending on what direction the storyteller wishes to take. Maybe it changes over time as Gehenna approaches.
The ritual is such a major thing, after all, you are trying to break a curse laid down by the creator. The chronicle contained in the book is described as, among other things, "world-shaking" and an "apocalypse plot".
According to early legends, it is impossible to escape the Curse of
Caine. Other sources claim that such an event is inevitable, but
usually posit a prophecy that it heralds the ends of the world. When
the impossible becomes real, it is a sign that reality is unravelling,
and even more horrible events - such as the return of the
Antidiluvians to the world - might be equally possible.
So there you have it. I thoroughly recommend tracking down a copy of the book and giving it a read as it seems to answer your question nicely, but fundamentally if you are running the game then it is up to you.
... you're really creating your version of the world's inexorable
slide towards destruction.