Why [Vastilok]? it's not a great gunblade, why waste time on it at all?

You are right. This gunblade is not as good as [Stropha] or [Redeemer Prime]. But I see this as a challenge to think a little bit out of the box. It's not considered "Meta" to use gunblades as Combo based melees, However [Vastilok]'s heavy build does kinda suck because of it's low critical stats. No matter what you use, it will be far from optimal for heavy attack builds.

Why didn't you max out your 60-60 mods?

For some reasons, DE decided to tie the number of elemental damage to it's frequency of procing. So while I could have max out those mods, I decided to keep their number lower than slash (Both normal attack slash value and it's projectiles).
Viral-Slash builds are great. They are effective and deadly to most of the enemies out there. But you also need to remember that Viral on itself is not going to deal damage. It is slash that does that! Viral is there only to boost the damage of slash!

I don't have [Primed Fury]. How can I get it?

[Primed Fury] is a primed mod that can only be accessible via daily login rewards and tributes. Upon 200 days of logging in, you will be rewarded with a choice: Primed Vigor, Primed Shred, and Primed Fury.

So what can I choose instead of Primed fury? I am very new/ I chose another mod

It is perfectly fine. [Berserker Fury] is still a very viable choice even after it's nerf. The only reason I've chosen [Primed Fury] was that I prefer having a stable Attack speed. Another good alternative could be the [Quickening] mod Which gives you 40% attack speed and extra combo counter chance. It's not as good as either of those I previously mentioned but it will still carry you through stuff.