Well, you may have to farm them again each NG+ in order to quickly dispatch Vendrick. As long as you keep 5 of them in your inventory, you may pop any extra one for they'll not give you any benefit whatsoever.
"Lower Vendrick's defense bonus when souls are in your inventory." OH, COME ON !! IT TOOK ME HALF AN HOUR TO KILL THAT BASTARD when I keep reading about things on the wiki, I feel even more stupid about everything I've read.