the only problem with that is the helicopter becomes a sitting duck. if the baddy has a crew, one of 'em might have rockets, and next thing you know, the pilot's got a real bee in his bonnet.

would be a problem likely to keep the helo steady enough for a shooter to work from. 

 Those do sound like overcomeable problems. For instance there
might be a way to install a longer range weapon capable of eliminating one man with
sniper-like accuracy. It could be aimed partially or fully by computer.
Then too, human snipers have to deal with wind, windage(windage is a minute problem
in modern war of course or at least I havn't heard many complain about it), terrain, etc.
I am not saying the objections mentioned are not valid. But there might be enough advantages
in the idea to try and overcome them.

Much of the snipers power and battlefield effectiveness hinges upon being undetected.  Putting him in a large, loud, flying object in an empty sky eliminates that advantage.

 

It would be easier to frankly just plan ahead and plunk said sniper down in a tree, building, hill, somewhere close to where you need him.  If the terrain is without any of those features, tell me why you need a higher vantage point.

Has been thought off and is trained for.  I asked someone who has been through the US Armys Sniper School at Ft Benning.  He even said it forms a task in the Sniper Competitions held at Benning attended by most NATO forces and a few other selected invitees.  

Why not?

 

US Coast Guard did it with the MH-90 Enforcer. They used a sniper to shoot the outboard engine of drug runners.

 

Saw the competition on the Military Channel.

They had to deal with all the problems noted in the thread (vibration, down draft, etc.) and the problem of firing downward at a steep angle.

 

There appears to be a lot of luck involved.  As I recall the team that scored highest on the task had never trained for it.

As previously mentioned the USCG does this sort of thing all of the time.  Normally a crew chief will take the shot me thinks...  I am sure that this technique has been used elsewhere to ventilate bad guys.

Maybe you could put the sniper in a powered sailplane. This would be very quiet and silent.

 

A sailplane cannot hover, but must orbit the target area, which makes lining up a shot very difficult.  Most sniper shots from helicopters seem to have been when the helicopter is either in a stationary hover (no motion) or pacing a moving target (no motion relative to the target).

 

Sailplanes are also much less maneuverable (due to the large wing span), so it is a not a good idea to operate at low levels like a helicopter.  Operating at higher elevations means longer range shots and/or firing down at a steeper angle.

 

Not to say that won't be done eventually, assuming it has not been done already.