One of the most versatile ways of clamping irregular -shaped parts is with use of soft jaws.
In this one I had to machine a triangular-shaped part from two sides.
It is going to be some sort of a part holding jaw for a robot.
So step one: Machine one side of the part in vise. hold on to 1/8" of stock. So make sure to cut your part on a bandsaw oversize.
Step Two: Bolt soft jaws to your vise and machine a pocket using outside contour of your part.
Be sure to relieve corners.
Step three: Clamp your part in the soft jaws and machine the second side of your part.
One important thing to consider is: this method is not very accurate. depending on the size and a shape of your part you may be able to hold it within 0.001" though.
See attached photos of the steps below.