Nidhogg is of Midgardsormr's First Brood.
The seven eggs came to Hydaelin at the same time. We don't know the order of hatching or if it happened at the same time.
It's unlikely, in my opinion, that their age would be that far apart to create much of a discrepancy of power (well power based on age anyway).
We have no evidence that Vishap was of that brood.
As far as we know, he's more likely to merely be Nidhogg's champion or something akin to a Dragon general.
So I'm pretty sure he's the underling here. They definitely would've made it more clear if he was one of the 7.
Nidhogg was definitely stronger, but had a lot going against him. See:
Also, we don't really know much about the real Bahamut to say how good he is.
The primal we do meet is not the real Bahamut as said in multiple comments above.
