do the cards light up (single red light) when the bios screen loads???
you are using on-board graphics? and have disabled crossfire?

nice hash I find a PITA.. you might need to add opencl 1 option..
I would swap cards and risers about and see of the problem follows the swapping.
To test i use cast-xmr,  you can run one instance per card. option --gpu then the card number

if your Vegas are all the same windows shouldnt need to install the drivers again when you swap about..

keep us posted