Yes, LiFePO4 does not work the same was a LiCoO2. When you start discharging them they will drop to about 3.2 volt very fast and stay at that voltage until they are empty.
Here is a full test of a LiFePO4 cell: http://lygte-info.dk/review/batteries2012/Unknown%20RCR123A%203.0V%20500mAh%20%28Blue%29%20UK.html
How are the sliders on the VP2. I stopped using my VP1 for 18650 because trying to pry back the slider and holding it in place while the cell is inserted became too much of a pain. I now mostly just use side by side SP2s.
“The slots have raised battery connections, i.e. flat top batteries can be used. The slider works very smooth and can take batteries from 30mm to 71.2mm”
We can’t compare the 2 chargers since the Opus is an analyzing charger/discharger, including facility for Nimh, while the other one (VP2) is a pure Li-ion charger only, (full list of functions not yet known) for the ‘VP4’.