Except that the ashlanders have no standing anywhere, and vivec will have no consequences exacted on him just beacuse a bunch of nomads said he committed the crime. Only someone with so kind fo real influence can make sure he is punished, and those people are all either long dead, or went to Akavir.
Punishment is a crude idea whose only use is to make the person who feels wronged feel better, with the side notion of hopefully deterring future acts of the same kind by the individual being punished and to make them sorry for what they did.
Since frankly I don't care about the first part, I'll address the second first and why Vivec requires no punishment for such a reason. First we must define what Vivec did wrong, and he does that for us:
"The sin of the Tribunal, however, is in the breaking of an oath to Azura to forebear from tapping the Heart with Kagrenac's tools, and in the folly of seeking to become gods. Breaking the oath was evil. Becoming gods was folly. If we sinned, we have paid the price... But, for the respect I held for Nerevar, and the respect I held for myself, I should never have betrayed my oath. Of all my life's actions, I most regret that failure."--Vivec
Now you might throw in persecution of the Dissident Priests as well, but he's already been reconciled with them, so we move on. I think we can rest assured that Vivec won't be breaking any more oaths to Azura, and with the Heart gone he won't be tapping it again, meaning that he can't do those again anyway, so punishment won't/can't deter him from it anyway. Also, he notes that they've already paid their price...
If punishment is to make him feel sorry for what he did, he already said he regretted it, and goes on to say that:
Now that Dagoth Ur is gone, we can turn our energies to the more humble needs of the people. It is good, honest work, and I believe there is redemption in it.--Vivec
So he is seeking redemption, or plans on it. You may argue that he went to Akavir and is skipping out, but really, can you say that there is no way that he's working behind the scenes... Vivec admitted he did, he feels sorry for it and isn't going to do it again, he did the best he could and is seeking to reconcile it; give him a break already...
A final note:
"The most I can say is: I did the best I could, as I saw things. Can you, mortal, presume to judge the actions and motives of a god?--Vivec