So, here's my take.

It has always been that the soul of a vampire is housed within the Kindred's heart.

If a Setite performs the Serpentis Discipline: Heart of Darkness ... if he/she wanted to be a real jerk, could literally commit Diablerie by drinking the soul of another, right out of the heart in his or her hand [remember, destroying the removed heart instantly kills the vampire].

As such; a Nagaraja could commit Diablerie by consuming the heart of a Vampire it was feasting upon.

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