I do not consider the Bomb in Fallout 3 to be a Nuclear explosion, it only took out Megaton if i am remembering correctly. and the Mother ship Zeta DLC thats like alien weapons and tech that is different lol.
I'm not sure how it doesn't qualify as a nuclear explosion, considering that it's an explosion, and the bomb that exploded is nuclear, hence why they call it a nuclear bomb and why it irradiates the vicinity of Megaton once it explodes, it also looks like a nuclear explosion. Sure, it only blows Megaton itself up, but that's probably just because Bethesda figured that destroying everything in the surrounging area too would be a little excessive.
does Archemedes count? I think it does.
Seeing as it's not nuclear or an explosion, I'd say no.
Plus, even if something that isn't actually a nuke could still qualify if it did enough damage, since it can't permanently destroy anything, just kill lots of enemies, I'd say it doesn't count.
But yes, it does seem like there's no real nuclear explosions (Not counting mininuke explosions, of course.) in New Vegas, but that's fine too. Just because past games have them doesn't mean it's absolutely essential that a new one does as well, and I don't think a post-nuclear game necessarily needs to have any nuclear explosions involved in it either, after all, it's POST-apocalyptic, meaning the apocalypse has already happened, thus it's part of the backstory, not the main story, and by now, I think anyone who knows about Fallout, even if not well versed in the setting's lore, at least knows that there was a nuclear war involved.
So yes, the game does somewhat break the series' tradition by not having a big nuclear explosion in it, but I don't think it's that big of a deal.