Promising DBZ work!
Certainly not bad for a first try! Graphics are actually not too bad, and the Japanese sounds are much appreciated. I happen to love that Don't You See song! Good choice of Japanese-ish camera angles too, and it's nice and long!
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I'd say your biggest work, aside from improving your animation as you continue your work (it's kinda annoying now), is working on a more compelling story. Vegeta going evil and killing everyone is really nothing new. Also, it's a bit confusing as it is... why is Vegeta a wierd modified SSJ4, and why is Goku only like SSJ2 when he fights him? And do you plan to explain all the wierdness in the rain? Also, your ending scrolls back into your beginning, which is kinda annoying.
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Well, enough ranting. I really like the idea of a continued DB/Z/GT fanfic in Flash, and I'd love to see how your work improves!