this quest is ours and ours alone

Dogs have comparatively weak eyesight and rely much more on their ears and noses for information. Vander largely keeps tabs on the Undercity through what he hears from people who are out and about (since he seems to spend most of his time at or near The Last Drop) and through his instinct for things that "smell" off or don't seem right. He can be observant at times, but there are several giant issues in front of his face that he totally misses.

While he knew the Lanes were restless, he didn't seem to realize his authority had eroded to the degree it had by the time Sevika and her allies defected to Silco. It's also debatable whether he sees the true extent of how deeply disturbed Powder is, since they barely interact on screen.

Vander doesn't really have an equal in the Lanes. Benzo is the closest thing he has to a sidekick, but he's not at Vander's stature. Even Grayson, who spends most of her time topside, knows this. Madam Babette seems like she knows what's going on around town, but doesn't appear to have much influence outside of her brothel, especially after Silco's takeover. Set aside the whole debate over whether or not dogs really have pack alphas for a second. To the extent that they do, Vander is clearly it. We don't see him hold that position for long before Silco challenges him for it. (You could interpret their initial falling out this way too: a struggle to see who would come out on top leading the pack in the direction *he* preferred.)

Last, of course, dogs are famously loyal. Vander feels protective of the whole Undercity and won't sell out any of them to the Enforcers, even if he knows privately that that person did wrong. He even offers himself as a scapegoat in an attempt to keep the peace when he realizes Vi was planning to do so.

His loyalty was reflected throughout the Undercity, as you see random people on the street covering for each other in front of the Enforcers with equal tenacity. The later culture of chronic betrayal didn't fully emerge until the era of Silco, the backstabber-in-chief.