IZOD Units, kJ/m^2 to ft-lb/in
To build a testing apparatus per ASTM D256 specs, you will need to get a copy of the standard first. You can purchase on ASTM's website directly.
Secondly, it will be extremely difficult to calculate the impact forces for a dynamics case like this, because it will actually be an impact history (force variation over the course of the impact). And this history varies from specimen to specimen.
The forces needed for the vise to clamp down the specimen firmly will be a function of the surface property of the specimen. Why don't you design some sort of mechanical locks to avoid the friction variation?