Gamescom is a much more Vita-friendly arena than E3, with Killzone Mercenary's trailer showing off its touch-screen prompts for melee kills.
Not much shooting was shown, but arrow prompts showed off the touch-screen kills, for swipes to use neck stabbing action and other melee-based attacks to take down enemies.
Since it was just screen-capturing, Sony hasn't shown off how any other of the game's controls would be handled, but we know for sure that if you want to murder a guy with your bare hands (or a knife), you'll have to use your bare hands to actually do it.
Very little information is out in the aether for Killzone Mercenary, namely the release date. Expect to see it some time next year for PlayStation Vita.