If burial is necessary for resurrecting someone as a vampire, then it feels like it has to be the pose at the moment of resurrection - i.e. the moment when the fate of the deceased is sealed.
But now that I think of it, I won't put it behind Cazador to arrange and pay for Astarion's funeral.
First of all, ideally anyone preparing his body for the funeral would take note of very aggressive and visible bite marks on his neck - and of the obvious fact that he was drained of his blood. That just screams "Vampire!". So, either the coroner was in Cazador's pockets and hid the bite marks, never mentioning them in their report, or Cazador just went ahead and started playing the benevolent noble who felt bad for the fellow elf meeting his death so early.
"Poor boy, he was so young and had no kin to take care of him. It would be my solemn duty to assist with his burial, this is the least I can do for him. Oh, don't open the coffin, please. The cruel things those vandals did to him...It's not the sight for the faint-hearted..."
That way, he would have had total control over the burial process (or the lack of it) with nobody paying much attention, so he could just chuck Astarion into a coffin without even trying to make him look presentable for his funeral and dump him into the ground, with tombstone as a formality.
Second, it would have allowed him to spin an even better anti-Gur narrative: look at those savages, mauling a servant of justice to the point he had to be buried in a closed coffin. How we can call our streets safe with such a cruel mob roaming them?
He possibly even succeeded with pushing the Gur community out of the Baldur's Gate entirely, which led to them settling down in Rivington.
Also, nobody would have objected to Cazador taking care of the dead magistrate, probably. Judging by the state of Astarion's tombstone, nobody had been looking after his grave. That means that he either didn't have any family members in Baldur's Gate (or probably ruined his relationship with them so badly, they never bothered to check up on him) because it shouldn't have been a problem for an elf to visit their son's grave every 50 years or so.
(I feel like I'm making Cazador more competent than he actually is, tbh)