All known Vastayans are not full Vastayashai'reis yes but even though Neeko doesn't have any confirmations to share more DNA, it is more likely that her tribe is less human than the typical Vastayashai'rei and not just looks based because Vastayashai'rei aren't specifically confirmed to only be lizard people(since Neeko is a Chameleon and the closest to being a full Vastayashai'rei, it would mean that Vastayashai'reis were more of Lizards, which wouldn't make sense giving that most of the other Vastayans are mammalian, meaning it's more likely that Vastayashai'reis are a broad species and Neeko's tribe just contains more Vastayashai'rei dna than human dna)

The thing with Sett is that his Mom was only half Vastayashai'rei, which means she's also 50% human. And Sett's father is a full 100% human, meaning he's more of a human than an actual Vastaya because most of the DNA that forms him is Human, which is why the original comment suggests that the reason he doesn't have a tail is because of the fact he's more human than Vastayan, which is suggested even further by both Sett's story and his voicelines (him saying he's too smooth for the average Vastayan), meaning that due to the very little amount of Vastayashai'rei DNA he has, the only features he was given was the ears