Try putting the end of the crank (where it's threaded for the pedal) in the vise and then insert a steel rod ( of the right fit to slide ) into the hole where the spindle goes. Then apply the required pressure on the rod to straighten the crank. If you don't have any rods of the correct diameter you may mount the BB spindle (better, use an old worn out spindle) in your crank and use that pipe (as in note above) on the spindle to apply pressure. This way you don't have to avoid the chainring when you use your vise. Leverage is an amazing thing. Steel cranks will take this kind of management without a problem. Aluminum? Not so much...