Kojima Productions aren't porting Metal Gear Rising: Revengeance to the Vita because they believe that Sony's handheld isn't powerful enough to support the Free Cut mode.
One of the unique gameplay features of Metal Gear Rising is the ability to cut through any enemy or object, at any angle, and watch your target fall apart along your H.F. blade swipes. Creative producer Yuji Korekado told Eurogamer that "as of now we aren’t considering a Vita version. The reason is one of the main appeals of Metal Gear Rising is this option of the free cut. That requires a lot of hardware power, which is only possible with the power of a PlayStation 3 or Xbox 360.
"So as of now, we haven’t contemplated bringing it to handheld consoles."
The Free Cut mode certainly appears to require a fair amount of processing power, especially since objects such as cars, watermelons, tanks and opposing cyborgs deform (and dismember) realistically. You can read more details in our Metal Gear Rising: Revengeance hands-on preview.