Hmmm…got some more info on this...
@Herms:
OK, so I just got that issue of Jump myself. The name of the game is "DragonBall Z x One Piece: Battle Taikan Gomu-Gomu x Kamehameha" (real catchy, isn't it?), and it's for the Lets! TV Play system.
In the game Luffy is trying to collect the dragonballs to restore the world to normal (perhaps meaning to seperate the DB and OP worlds?). You can't quite tell from the scans above, but the people Vegeta is fighting are the Skypia preists, and the caption implies that Enel is straight ahead. The captain to the Oozaru Vegeta picture says that Vegeta is being manipulated by somebody.
So…it appears to be a Plug 'N' Play game.
EDIT: Strongbad beat me to it.