Why didn't Netero use a limitation and vow like Kurapika to beat Meruem? Kurapika was able to chain the Phantom Troops by making a vow that she would only use the chains on the Phantom Troops or else she would die. Why didn't netero do something similar?
Simply put, one can't just add a Limitation or a Vow to an ability.
Specifics of developing a Nen ability are left a bit vague in the story, but it's rather clear that Vows and Limitations are inherent part of each ability that are added to them as they are created. And since Netero didn't create any new abilities to fight Meruem, those would have needed to be already present.
Also, most hunters dislike adding Limitations to abilities. The enhancement of power they provide is inconsistent and the conditions need to be very severe for the benefit to be significant. Kurapika is able to use as severe Vows as he does because hunting the Phantom Troops is literally his life's work. He's willing to sacrifice a lot of the general power of his abilities to make them really powerful in specific situations.
Now, there is one powerup that Netero could have perhaps used. General emotion of risk. This is sort of the basis of where Limitations and Vows get their power in the first place, but more primal. It is the risks that Limitations impose that give them their power and it is possible tap into the same source without imposing a formal Limitation. Shoot used this when he covered one of his eyes while fighting Youpi, sort of imposing a temporary limitation on himself. It could also be argued that Gon's Jajanken gets a bit of a powerup from the risk that its long windup and general vulnerability present.
Of course, Netero was already going to fight possibly the strongest living being on in the known world, so I'm not sure how much of an extra risk there could be.
Side note: I haven't read the Dark Continent Expedition arc or the Succession Contest arc, so if there are any exceptions to these, they'd be there.
Netero had been alive for a long time. This was the first fight in a long time where he was actually going to go up against a worthy opponent. As others said previously limitations/vows can’t really be created after the ability has been conceived except in cases like with shoot.
The only thing that Netero could have possibly utilized in the form of a vow/limitation is some form of Post Mortem Nen. But it is entirely possible that even that might not have worked because he technically already had a Post Mortem ability in the form of the miniature rose, and knowing that he had that, he might not have been able to generate a conviction strong enough to go off after he died