For now Victini is an obtainable Mythical in the game
Heres some directions on how to get him on your team! :D"wow that guy has no idea how to get victini." here's how to get it. After defeating the 5th gym, go to the temple. Next enter the ruins. you'll have to use a pokemon move called rock smash on it to proceed. go to the room with the 3 color gems. enter the open door. you'll see a bunch of rocks. underneath one, you will find a hole. then somewhere in that room, you will receive your first gem. Ready for the tricky part? place the gem into the kings statue. "a rumble can be heard from deep within the ruins. Go back to the 3 gem room. another door opened. go in the door. a puzzle can be seen ahead. it will tell you directions, with pictures. get rid of all the blocks. It is VERY tricky. so take your time. when you complete it, you'll receive another gem. go back to the king statue. "another rumble can be heard deep within the ruins. " another door opened in the gem room. find a torch and light it on the rope. go toward the coffin. You will encounter the king as a mummy. you will have to battle him to get the last gem. go back to the statue when you got the last. "the final rumble can be heard deep within the ruins. So you know the corridor to get to the gem room? In the corridor, a door opened. Go inside the door. There will be 2 pokeballs. one with a nugget. one with a giant nugget. go to the victini painting on the wall. You should encounter him. I recommend using master balls on Mythical Pokemon. Hey its just me, not you. There's a simple way to get him. or.. the only way!
No matter if you catch, faint, or just run from Victini, after the battle, if you forgot to save, you can still find Victini roaming in the wild.