To get the hadoken fire ball in the game you must have all heart tanks, sub tanks, all body parts, and beaten all the bosses (not including the sigma bosses) Then go to the armored armadillo stage and go through the stage without losing a life and taking a hit. Best way to do that is to fully charge rolling shield and go balistic with the storm tornado. When you are about to get to the boss gate with that spike board, when it is about to fall jump off to the energy tank, there will be a capsule with the hadoken power up in it instead of the energy tank. Dr. Light will then explain to you about it and jump in the capsule to get it. The hadoken fireball is the strongest weapon in the game and can kill any boss(including final sigma), but you can only use it with full HP. So use it with a charged up rolling shield. To use the Hadoken Fireball, press down,diag. right, right circle, or if you ever played street fighter, it is the same way to do it in this game. X will then shoot out a blue ball from his hand and say Hadoken.
To earn Zero's Buster, you must not get X's buster upgrade. Once you are going to fight Vile for another round, he'll be himself. He's a jerk when you know what I mean. He use Cherry Bast (Machine Gun), Shoulder Cannon (Adjustable cannon mounted on his shoulder) and bomb. The machine gun is easy when you get him first. The shoulder cannon is only hit by diagonal way (Vile can get a perfect shot if he jump.) and a bomb which is easy to dodge by jumping over. Once you fought Vile. Zero will reward you with his dying words with a buster. Which unleashes red colored shots. Still, you can charge special weapons. But the firepower is twice power up than X's upgraded buster.
The Hadouken (similar to Ken and Ryu's type) can deliver a instant death blow to any boss (Sigma's dog, himself too.) But, I'm not sure about his robotic form.) Even the toughest to the weakest boss. You'll need it the hard way.
Complete the game to unlock these:Play as Vile - Must be as X
Megaman X Anime Movie - Must be as X
[b]New Title Screen[b] - You will see X and Vile surrounded by flame