FrogReaver
The most respectful and polite poster ever
I agree with level 1-3 if we are just counting single target damage. Cleave is likely much better at this stage for damage, though it can be a bit DM dependent on how they position their enemies. You also lose out on useful Masteries like topple that don't up your damage, but instead increase your Battlefield control and possibly aid your allies damage.
Level 4 with feats like GWM, PAM, Sentinel that all can give extra attacks, I think it's much harder to say this. So much will be assumption based that i don't think the question can be definitively answered.
For levels 5-8 that puts the Nick Barbarian down .5 damage (not to mention feat options acquired at level 4).
That's a 1 time per turn ability, that already has nearly a 100% hit rate with 2 reckless attacks. The third attack does add a bit of accuracy here +2.2% assuming 60% base chance to hit. That's not enough to make up for the 0.5 gap.
That said, for all intents and purposes they are dead even from 5+ without feats. It's just feats likely favor the non-dual wield barbarian.
The vex tactic is certainly a defensive concept for a Barbarian. It's a way to reckless attack less so the enemy doesn't attack you with advantage so often. I'd probably pair it with a subclass that grants extra damage.
to try toppling or run with my sap mastery if I am mobilized into isolation. Or vice-versa.. start reckless to draw fire, then become a brickwall after presenting as an easy hit. Get the free shield mastery shove for battlefield control. The reaction to 
% negate dex-based saves pairs very nicely with the barbarian advantage to dex saves. Even when he starts to lose life, the bonus action d12 healing goes a long way. If he needs to enter cleanup mode, or if I need to screw with the DM a little, then I opt to go reckless and become more of a dmg-merchant. This character can break grapple easily as well, especially if using the warhammer + 🛡... then you have multiple shots at shoving without losing out on action economy. Good that Zealy ability isn't tied to reckless attack any more. The other subclass I would choose with for this is Wildheart... that bear option is tempting, but then you are minimizing dmg potential even more.
