I always liked the "Beam whip" from CV64, so I think Kaz's theory is interesting and has a little credibility behind it.
The Belmonts were trained to fight evil all their lives, they knew they always had to keep fighting and could never stop, so they limited the power of the VK to ensure they can always keep fighting. Also, because they knew they could complete whatever tasks at that stage, because confident BAMFs.
However, Reinhardt and John were not trained as such. Were not confident in their battle prowess and felt they needed the edge of the ultimate VK form. They also might have been more concerned with the immediate threat of Dracula and not the long term battle against the dark forces.
I mean, hell Richter has item crashes and shit, Reinhardt and Jon had just the whip and sub weapons (hell, Reinhardt also carried a small sword for additional protection!). Reinhardt and John also worked their bodies (i.e. BEEFCAKE compared to most CV whip wielders of similar vintage) since that's all they really had to help them combat the dark forces.
I like to also imagine Reinhardt lived a long happy life with Rosa. Maybe she helped him not lose all of his life force? I dunno...just some musing from me.
