The Vong were a lore compatible addition to the series, and enhanced understanding of the themes explored in the original trilogy.
Yes, the Vader story arc's closure and significance seems to be subverted, but in actuality that story remains a great fiction, and the Vong events do not diminish the significance of what that story depicts. It's really the audience reaction that was a negative misunderstanding of what they naively expected from fiction.
The Vong story grows Star Wars lore from being merely a comic book, naively imitating legend, and brings its lore into a more realistic and maturer reflection of our reality. Heroes fail, tall people from servile races are more likely to become living targets, and China is a distant land that represents a real existential threat to our own. It's also consistent with overall lore, with events proceeding from those from the Anakin arc. Heroes fall and fall short of their potential glory; that's what Episode III was all about. You don't hear more people complain about Anakin not simply defeating Palpatine while still an unsullied Jedi. But if Episodes IV-VI were released after Episodes I-III, you most certainly would, just as you hear fans complain about heroes in DBZ not living to the full potential of their destiny. These are all results of naive audience expectations, and fans shouldn't cling onto simplistic illusions about story, but should allow such challenges to their expectations to help them mature as an audience and as people.
And the children of Vader storyline still restores balance to the Force, even considering the events that followed. Sidious's corrupting of patterns in the Force across the galaxy were weakened, when Luke and Anakin defeated him, and revived the Jedi Order. The galactic civilization would have suffered more centuries of darkness from the ultimately suicidal brutality of the Sith. While I myself sometimes joke that Sheev did nothing wrong, think of it this way: Two Siths extendedly vying with each other, with no serious foe to unify their objectives, would have increased competition and resentment of the other, which their emotional imbalance would have acted on, and increased the viciousness of how they'd deal with each other. This would have led to catastrophes far more chaotic and annihilative than Alderaan, had not the events involving Luke, Leia, Yoda, Anakin, and Sheev, happened to correct their course. The fruition of the restoring of balance to the Force is an excellent story that explores powerful themes about goodness, justice, and balance, and its struggle against disorder. That story will remain a strong source of inspiration in fiction. The appearance of the Vong doesn't diminish the essence of that part of the story, even if fans mistakenly feel that it reduces the feeling of closure. But reality is never that simple, and we shouldn't expect fiction, which reflects it, to be so either.
Continuing in this sense, note that the Vong also are a realistic reflection of unexpected events in our history. The Mongol invasions disrupted many societies' expectations. This is an unpleasant fact, most agree. But should we sweep such events under the rug? No, that would be suicidal foolishness. To do so would to ignore the potential of surprise in reality, and would be a self-defeating fallacy that would only leave us vulnerable to the next civilization-threatening surprise, like that of China influencing the rest of the world, that the West has exploited, neglected, and allowed China to. For the narrative bridge to them, while Thrawn having encountered them often feels contrived for fans, the writers still wrote them consistent with lore, even through inversion. And Thrawn was a worthy rival for Vader, just as the Vong were worthy opposition to the Jedi. Remember that inversion of Force properties occurs often even in the film entries. It's not necessary for me to detail methods to block lightsabers and Force powers, but those who oppose the Vong stories should ask themselves, why should subversion of the Force be limited to technology, and be impossible through biology, as with the Vong? Their's is a narrow complaint that has no real defense.
I present this on a minor corner of the internet, rather than one of the big forums or other media, because chans allow more freedom to communicate facts, and a higher degree of reflecting reality, with the rest of the internet censoring free speech and promoting dysgenic liberal ideology. I ask my fellow Star Wars fans to consider my message on how fiction can broaden our perceptions of reality. And to be persuaded to see that the Vong are an insightful addition to the franchise, and more than just a cynical financially motivated exploitation of the franchise.