Does the yolk sac in general form always at the vegetal pole?
If so, is this the case for Danio rerio (the zebrafish)?
Generally, yes, yolk is at the vegetal pole. There may be exceptions but I am not aware of them.
You can read about zebrafish (Danio rerio) embryology in great detail in this paper. Figure 3 from this paper (see image) shows the immediate post-fertilization zygote, in which the animal-vegetal axis is first set up by the migration of non-yolk to the animal pole.
UPDATE: very explicitly labeled yolk figures.