I will concede that both Ventus and Kairi have pure hearts. Kairi because she's a Princess of Heart and Ventus because (SPOILER)Xehanort extracted his darkness to make Vanitas. That is true. However, before (SPOILER)Xehanort brings Ventus to Eraqus, they stop on Destiny Island where a light of a new born heart comes to Ventus and heals/joins with his broken heart to repair it. Also, stating that eventually he will get back the part that he lost (i.e. the part in Vanitas that contained his darkness). Most likely this heart was either Riku's or Sora's. Given the fact that Sora's Heart is the one that is said to connect all Hearts that he encounters as well as the fact that Ventus' Heart takes refuge in Sora after the end of the game, I'd say it is a safe bet that it was Sora's that healed Ventus' heart. Plus the fact that Aqua, upon landing on Destiny Island, was struck by the fact of how similar Sora and Riku were to Ventus and Terra, to the point of worrying wherein she made Sora promise to always be by Riku's side and prevent him from straying to the Darkness. Oh, another thing, Roxas is practically a spitting image of Ventus.

I am not saying that Aqua and Sora don't have things in common, they do; however, the similarities between Sora and Ventus are staggering and much more numerous than those between Aqua and Sora. The connection between Aqua and Sora is very much a tenuous one compared to that between Sora and Ventus.