Just did this today. Pic 1 shows how the clip can get tightened in such a way as to leave the "grabbing area" too loose. Okay that's terrible tech language but I'm sure you've encountered this so you know what I mean.
Pics 2 & 3 show the clip placed in the vise before tightening to close the grabbing area. Give it all ya got.
Pic 4 shows screwdriver inserted [hammered in] to re-open the clip just at the bottom. This combo, clip and screwdriver, goes back into the vise for tightening again only this time it tightens to reclaim the original functional shape of the grabbing area. You will need to remove any wood panels attached to the vise to ensure that only the area between the rounded top and the screwdriver get clamped - you don't want to be tightening down on the screwdriver itself.
So far I've been unable to move the L-arm in the clip by hand. Will report back once tom is in place...[still working this]
Mitch