From testing that has been done, staking VIT at 50 has almost zero noticeable effect. DEX increases shield block rate, but again the rate is so inconsequential, you may as well not even waste the points in it.
STR increases your WS damage and by extension, your enmity when using The Fast/Flat Blade combo.
MND is probably the single most important stat to put your points into.
EDIT: Edited for some mistaken info.