Snapped the edge off

While attempting to square up the bottom of the tang on a Wade and Butcher I had it clamped in the vice between two pieces of 1/2 inch plywood I heard the sickening sound of the edge snapping off completely. On y second pass with a fine cut machinest file.

What kind of clamping set up do I need to keep the edge attached while working the tang or working the back with a file?

I've spent the better part of two hours searching for the remady of clamping up the blade, and haven't found it yet.
Thanks

some sort of not too hard or soft rubber, or maybe some pliable putty?

I like to use soft thick leather on both sides of the blade. Pink pearl erasers make nice soft jaws.

Charlie