When Almalexia and Sotha Sil fought Mehrunes Dagon at old mourhold, vivec wasn't there to help out.
When Almalexia and the Underking fought Ada'Soom at red mountain, vivec again was nowhere to be found.

He did help out against Ada'Soom. He made everyone breath water and then flooded Morrowind.

he also stop that big evil rock from crashing into vivec city,

though we know what happening to that in the end

and he came up with the plan to defeat dagoth ur while the alma was plotting and sotha not giving a [censored] about anyone

Vivec was too busy unraveling the secrets of the universe and finding new people and things to put muatra in. Aside from which, remember this was before they began to dwindle and fade. He likely had every confidence that the other two could handle it.

There's a touching exchange in the Trial relating to this:

Allerleirauh: How is it that you spoke so seldom with your sister and brother, Ayem and Seht, allowing Ayem certainly and Seht perhaps to fall into madness at such a critical hour as that of the loss of the Tribunal's godhood? Any mere mortal would have devoted more care to his family.... your inertia surprises me.

Vivec: Is this a crime, Mercy? Or merely a stark variation that causes you a personal anguish, this difference in the kinship of snowmen and their devils? To admit I am mortal does not admit that I am a man. [...]
I did not speak to either Ayem or Seht enough before their death because…well, because, mortal or divine, that is the way it always is.

I have never been of the opinion that Vivec is really that nice a guy.

Remember, kids. Please CHIM responsibly.

Where were his siblings while he was maintaining the ghost fence?

Vehk is Vehk.

One of my favorite bits from the Trial.


Vehk gonna Vehk.

[Vehk] me? [Vehk], you buddy!

I [Vehk]ed your mother so hard last night...

It's been so long since I've had a good [Vehk].

You're out of your [Vehk]in' mind, man.

Vivehk once saved Sil and Almalexia from Dagoths. So, not always, but he helped other Tribunes.

Why would he approach two mad gods? If Alma would have killed him, things would have got messed up.

As others said, he flooded Vvardenfell. He also saved the butts of the other two when Dagoth attacked them, and then recovered Wraithguard.

EDIT: Let's forget about NUMINIT. [Vehk] is better.