I'm trying to pocket .1250 thick 4" X 6" piece of 6061 aluminum and I keep getting bad vibrations :-). I am using a new kurt vise which pulls the piece down and a goodly amount of pressure but the piece still vibrates badly while being machined. Any ideas?

Carpet tape the piece to a backing material or add additional clamping.
RICH

Or sandwich the piece between two sturdy sacrificial pieces.

Forget the vice for this job. Clamp it over the top of another pc. of alum, and set your z depth to -.010 approx into the lower pc.
If your cutting through.

Ed

What they said.  1/8" is just too thin.  You can glue it to the tooling plate underneath and it will be held over its entire surface.  I'm not too comfortable using only the glue, although many do, but it sure helps. 

You can use hot melt, super glue, or even Loctite with metal.  All work pretty well and will release with heat.

Not knowing what your part looks like, it's hard to suggest further steps.  For example, if you have some holes, it can be helpful to drill these first and then use them as hold downs as well.

Cheers,

BW

You can use jewelers wax to glue it to another piece of thicker metal.  It melts at 150 degrees F.  A very thin layer holds well. 

Vince

if possible use a vacuum table...