I diablerize a vampire with blood with Kyrylo.
When can he play Slake the Thirst?  Immediately after the fifth step in the 
diablerie chain, prior to the Blood Hunt referendum?  I assume so, since it 
clearly says in the rules "effects may be played either before or after, as 
appropriate".
I searched for this using this search criterion:
slake diablerie group:rec.games.trading-cards.jyhad author:lsj
Anyway, here are some relevant texts:
6.5.5. Diablerie
Diablerie is the act of sending another vampire to Final Death by drinking 
his blood. Only ready vampires can commit diablerie. Vampires marked as 
Blood Cursed cannot commit diablerie. The vampire committing diablerie is 
called the diablerist. Diablerie is resolved as follows:
    1. All blood on the victim is moved to the diablerist. Blood in excess 
of his capacity drains off as normal.
    2. The diablerist may take any equipment on the victim.
    3. The victim is burned (sent to his owner's ash heap). Any cards and 
counters on him are also burned.
    4. If the victim was older (had a higher capacity) than the diablerist, 
the diablerist can be given a Discipline. His controller may go through her 
library, ash heap and hand to get a master Discipline card to put on the 
diablerist and then reshuffle her library or draw back up to her hand size 
as necessary. This may increase the diablerist's capacity by 1, but does 
not automatically give the diablerist a blood to fill that new capacity.
    5. If the victim was Red List, the diablerist may receive trophies (see 
section 11).
The steps of diablerie are treated as a single unit. No effects can be used 
to interrupt the diablerie; effects may be played either before or after, 
as appropriate. After diablerie, a blood hunt may be called on the
diablerist.
Slake the Thirst [HttB:C]
Cardtype: 	Action Modifier / Combat
Discipline: 	Sanguinus
[san] [COMBAT] Only usable when this vampire gains blood. Choose another 
ready Blood Brother of the same circle. That vampire gains the same amount 
of blood from the bank.
[SAN] [ACTION MODIFIER] As above, but usable as an action modifier.
Artist: 	Jami Waggoner
Correct.