I am not really sure because I dont have vita.
But reading around I found this....
Posted by Sakyr
Here's a step by step:
1- Call cambell as soon as you get in the fight and kepp calling him till he tells you the second controler trick.
2- Lose the fight
3- Continue and go back in the room. Call cambell again and he will then tell you to shoot the statues, take out your famas and shot away it should only take a few bullets!!
4- PWN Mantis's A$$!! lol
Not sure if it is right or not but worth a shot! (Couldnt find anymore info on this subject)
how do you change ports on vita to beat mantis?
In the game, touch and hold the Vita screen until a list of options come up. One set of options is controller settings. In there, you can change the controller port from 1 to 2.