The PS Vita version of Dragon Ball Z: Battle of Z will have an update that will add Japanese voices.
Namco Bandai UK confirmed this via Twitter:
We’re happy to announce that there will be an incoming update to DragonBall Z Battle of Z for PS Vita to include Japanese voices!
If you’re sick of hearing the same voice actors that you heard all those years ago, you can switch to the Japanese voices if you want to.
They didn’t specify when the new Dragon Ball Z: Battle of Z update will be coming, but it should be very soon.