So what happens to Thinbloods when they diablerise another full vamp? And how do you see that happening whithout becoming a pariah in the vamp world? I understand it's typically allowed through Blood Hunts but nothing more.
As thinblood you're a pariah to begin with.
While V5 pushes the idea to play thinbloods, they're still not really accepted by other vampires:
p110 V5:"
Many Kindred view the thin-bloods as a threat to the
Masquerade and will destroy them, if given the chance.
The thin-blooded can be of any mortal origin and
are not affected by their parent clan’s curse nor by their
preferences in the same degree as their sire. If they are
allowed to rise, they will likely be turned away, gaining
the luxury of freedom and the danger of solitude.
In 2009 the Inner Circle of the Camarilla issued
an edict that all thin-blooded must be branded or
tattooed with the “mark of the crescent moon”. Inked
by a Tremere tattoo artist or scorched with a beam of
sunlight, the brand is irreversible. Even Anarchs and
unbound think twice before associating with a marked
Mercurian.
"
Also to offer a few more informations about Caitiff/Thin-Bloods, as people are prone to mislabel the informations we got so far; Caitiffs are vampires without a predetermined set of disciplines and without the specific bane of the established clans.
p105 onward
"
Not all Kindred inherit the curse of one
of the 13. Some don’t care what their
lineage is. Certain bloodlines of kindred
have always been clanless, and
sometimes childer of the clans are just born different.
They’re often discarded, exiled, or choose to distance
themselves from the clans that despise them. Pure
vampires, the Caitiff make up for what they lose in
respect and pedigree by flexible blood and the absence
of a crippling bane. Proud or ashamed of their clanless
nature as they may be, these creatures have no family
and represents vampirism at its purest and most
individualistic.
"
Rule wise they choose 3 disciplines they like as their starting disciplines and instead of gaining a sepcific clan bane they gain penalties in social interactions with people who know they're Caitiffs. And while other clans buy their clan discipline as Dots x 5, they buy their disciplines with dots x6 in experience cost.