X2
The biggest issue with u-joints is creating a bind when re-assembling. It's kinda hard to explain it in words, I wish I could show you...Bottom line is that when all is said & done, the joint should articulate with no resistance whatsoever. That is, once the clips are in, the joint cups need to be seated outward in their loops so they contact the clips - thus unloading the "X" part. Positioning the driveshaft loops strategically on (not in) the vise (like you mentioned) and applying gentle taps with a dead-blow or brass hammer on the loops of the opposing axis (both ends) will do it.... Again it's easier to demonstrate than to explain. Hope it makes sense!