This didnt happen to me the first time I played this game. But after being declared Nerevarine by all 4 Ashlander tribes I then proceeded to Aldruhn to the Redoran councilors to ask about becoming Hortator and one of them told me that I needed to talk to Saryoni in the High Fane, so off I went to Vivec's Temple and he sent me to Vivec who of course proclaimed me Nerevarine and gave me Wraithguard.

Having played this before, I knew Vivec to be my last stop before launching my campaign against DU. But I still havent been named Hortator by any of the great houses yet. Do I still need to be? Or has Vivec already given me what I need?

In reality, Wraithguard is all you need. The fourth and fifth trials are only to convince Vivec that once he gives it to you, it won't fall into the hands of House Dagoth.

The only real downside is that you won't get the Hortator items, which may or may not be useful.