Does the yolk sac for Danio rerio form at the vegetal pole?
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.