Standing vert and running vert have different mechanics IMO. In a SVJ you want to recruit as much power as possible, for some people that includes a deep bend (see: KF) others (like me) have very little or no bending at the waist.
In a DLRVJ...I think the folding is core weakness. You can't physically transfer the horizontal speed to vertical speed due to lack of stability and your body folds/bends/breaks at the waist due to these forces (when approaching at max speed on a runup). To this end I say watch Jordan Kilganon, Gerald Green, Young Hollywood, TDub they all keep a straight back, no energy/power wasted from unnecessary movement and they tramsfer from horizontal to vertical faster than those who fold. Basically, if you're folding you can benefit from a stronger core.