my theory on the kienzan is that the manipulation of energy is a basic technique that may simply have different names. when vegeta comes to earth he can fly, shoot energy beams that are like a kamehameha, create false moonlight which may not be that far removed from learning solar flare, and apparently either already knows something like the destructo disc or learns it from watching krillin do it. really the only difference between the saiyans and earthlings is that the earthlings can sense external ki(you'd think vegeta would have to be acutely aware of his own ki in order to manipulate it but hey, nothing else in DB makes sense), it seems like a lot of techniques are interchangeable or related