That depends...

If the "whole song" is done in the Vita MIDI editor, you can simply select all of the MIDI events and raise or lower them at will.  This, of course, does not change a major into a minor, etc.

If you have ANY audio objects to be altered in the arrangement, that isn't possible.  Your only option to change the pitch/key as a whole is to export the arrangement (whole or in part) as an audio file (WAV, MP3, etc.) and re-import it into a new track.  (Be sure to MUTE the previous tracks/objects)  Then, use the Time Processor in the track or object effects rack to raise or lower the pitch.  Raising or lowering the pitch more than a few semi-tones may produce an undesirable effect.  You can do this to a single object, a group of objects, or the entire arrangement.

NOTE: You can change the pitch of any single MIDI or audio object without exporting.  You only need to export if you have a mix of audio and MIDI objects or if you need to alter many audio objects at the same time.  You can do them one at a time, but there's a greater chance of error.  Also, you'll have to decide whether to include, or exclude, drums and percussion.