DC/DC converter/eFuse maximum output capacitance
Add the recommended ~830uF capacitance as close as you can, on the output of the Vicor power module. Make sure the ESR and ripple current ratings etc are all correct. Make sure the type of capacitor (Electrolytic, Cermaic, Tantalum etc.) chosen is correct and has the correct ratings and ESR as mentioned before.


Add smallish 47 or 100uF capacitors on the inputs of each of these separate PCB which are the 12->5V Buck converters. The datasheet for the converters will say what they want as input capacitance, otherwise as I said go for some kind of local supply. This will prevent current-lag related voltage sag/ripple when the 5V output goes under load, yet the 12V input has no power supply decoupling available. 


The load capacitance will be the capacitors you put after the efuses, because remember a discharged cap acts almost like a short circuit during power-up stage, so that will greatly affect the way the efuse works for "soft start" operations.


Therefore if you put a 100uF cap after the efuse, just before the 12->5V DC/DC converter, then use 100uF as the load capacitance for the efuse calcs.