Unfortunately, there has never been a portable Saints Row game, but it turns out that's not for lack of trying.
Kotaku:
Today on Highlight Reel we have physics guns, jumping jacks, lightheaded Fallout companions, hammers, and much more!
U64: "Some days ago, Volition talked about Saints Row: Undercover, the cancelled PSP game revealed by producer during one of their weekly Twitch Streams. Today you will be able to play the prototype yourself: the lovely people at Volition created a playable ISO for the Undercover prototype and shared it with us to be preserved and archived."
This is awesome, I hope more developers would release beta or uncomplete versions of old games that never got released or never will be released commercially.
They should revive it and bring it to Vita.
The question is will it continue the SR2 story and not go the route of the abortion called Saints Row three?
Revive it for the Vita.
Looks better then sr3/4.... Take the series back to gangsters paradise of sr2