The vacuum gun is the perfect (read grossly impractical) evil genius weapon, the victim simply disappears and the remains are disposed of later at the assassins leisure.
As I see it the vacuum gun would consist of a backpack mounted miniature steam engine running a vacuum pump to fully evacuate a pressure vessel. This vessel is connected to the 'gun' via a large diameter vacuum hose, like are used for railway braking systems, the gun consists of a pipe with a funnel on the end an a rapid opening valve at the other.
The vessel is evacuated and the steam engine allowed to idle (to allow a virtually silent approach), the funnel is placed next to the victim and the trigger (the valve) opened, the victim is instantly exposed to a perfect vacuum and 'disappears' through the hose into the pressure vessel.
I have the concept, now to build the prototype, next world domination! mwah! ha! ha!
Seriously though I now have a backstory / concept for my next build (hopefully in time for Asylum 2017, yes I am thinking that far ahead!), what do you think?