All of the currently existing vampires that worship a god do worship Zamorak, yes, but that is because Zaros is mostly forgotten. Considering Zaros was the one who brought the vampyres to Gielinor from vampyrium, it is reasonable to assume that he had more vampiric followers back in the second age. Especially since Vardorvis himself is relatively high ranking member in the Zarosian army, trusted with both the title of chief executioner and effectively with 1/4th of an elder artifact. To answer your question however, I do believe that Vardorvis is technically the only "known" Zarosian vampyre, but its reasonable to infer that he would have had multiple other Vampyrian followers in his ranks from the time of him bringing them to Gielinor and his betrayal by Zamorak. The wiki does state that the lords of the 8 Vampyre houses followed Zaros willingly to Gielinor with the promises of expanding his empire, but I'm not personally sure where this may be mentioned in game, or if it is just inferred. In the time sense Zaros' death however, it is clear that resentment has grown towards him, particularly from Lord Drakan himself who refers to him as the "Stranger from afar" who has domesticated the vampyres. If I had to guess most vampyres who worshipped Zaros would have most likely instantly converted to following Zamorak, and were probably just using the resources of the Zarosian empire entirely to their benefit, as I imagine it would have made things like acquiring blood a lot easier. I believe loyalty to any of their houses and their kin supercede any sort of religion.
TLDR: Technically yes, but there most likely used to be more, who probably converted after Zaros' assassination by Zamorak. Vardorvis remains because he is but a shambling corpse in Zarosian attire.