The story of a seemingly lower caste outsider who enters and saves an upper caste society from itself is nothing new (e.g. Thomas Temple Hoyne's 1934 Intrigue on the Upper Level), but here the story is much more one of mystery and intrigue than one of social commentary, particularly as Flyer society is little described, beyond very broad strokes. Similarly, there is no description of how it feels to be a Flyer, as opposed to a Lander, what does such an ability bring to the life experience of a Flyer? Edmond Hamilton's "He That Hath Wings" (Weird Tales, July 1938) is perhaps the best story to explore this experience. As for any mystery regarding the bad seed among the Flyers, what mystery is there? Very little: as the traitor amongst the Flyers is obvious early on with his "knowing smiles" and the fact that Larana/Aen shivers when in his close proximity -- amongst other clues.

Stories of winged humans date back to Daedalus and Icarus, but have made numerous appearances in fantasy literature, particularly in lost race tales, from Robert Paltock's The Life and Adventures of Peter Wilkins (1751) set in Antarctica to Stanton Coblentz's When Birds Fly South (1945) set in central Asia. Unlike these works, the Flyer's ability to fly is not exploited, either in terms of what they are physically capable or of any symbolism. Certainly the sense of wonder and desire that Paltock's and Coblentz's male protagonists exhibit before winged humanoids, is not matched by the Landers of Vassal of El, rather the Landers are mostly jealous and resentful of the Flyers. Even if Vassal of El's original title was Wings of Angels, there's very little of an angelic nature in the Flyers, a number of them being rather elitist not to say outright racist towards the Landers or "grubs." What is perhaps interesting is the parallels one might draw between the character of Larana/Ael, a wingless Flyer, and the flying women whose wings are clipped by men in the early (1914) feminist lost race novel Angel Island by Inez Haynes Gillmore (both authors coincidentally having been born in Iberian cultures, Oliver in Puerto Rico and Gillmore in Brasil), but I'll leave that to those better versed in such studies.

For an entertaining, don't ask too many questions read, Vassal of El delivers, and is suitable for readers young and old, which for a new author like Oliver is already a significant attainment. However, if you want your fantasy with something below the surface, perhaps some socio-political commentary, then look elsewhere.