I could only find one example of Vehk the god and Vehk the mortal in the Trial, but nothing about the god having always been. Though it appears to be a common legend on the internet. I suspect it was developed on the forums as a way to explain the historical validity of the Sermons, but that really is missing the point of the Sermons.
I believe comes from this:
“But when Vehk the mortal reached into the Heart, he ceased to be anything except for what he wished to be. The axis erupted. There was an exact cracking, an instant of pure Aurbis, his hands burnt black by that ever-nil of static change, and Vivec the god who had never been had always been. A whole universe swelled up to legitimize his throne... as the old universe, where Vehk the mortal still lapped up Godsblood, warped itself to accept its new equivalent. And like all things magical it simply could not happen, could not Be. Red Mountain was the intersection of the Is-Is Not as it was of old, its center point, and it did not hold. And so the Dragon, having broken, saw fit to heal, turning into the world you know. Except now Vivec the God was alive before his own birth, which had, in fact, really happened in the death of the last universe. Hard to grasp in three-dimensional thought? Why, of course it is. And so that is why some semblance of my anguished personal reconciliation found its way into my own scripture. Why did I leave the Nerevarine two accounts of his death, one that I could have easily erased from the minds of my own people? Because he is Hortator, GHARTOK PADHOME AE ALTADOON DUNMERI, my lord and king in this world and the last, and as Vehk and Vehk I murdered him, then raised him, then taught to him to know, and so would I have it when he came to me at last that he decide. I give you this as Vivec."
(The Trial of Vivec)
Unfortunately I can't find the original source of this passage though it has been quoted in a few different forums and in an article in the Imperial Library.