Vlad pool

So I really like vlad but I suck.... blood jokes aside, i recently "canceled" a Syndra ult with my pool, only to have her recast it on me when I came back up anyway. So what did I do wrong? Did I need to wait longer for her ult animation or do I have to take all 15 balls of it before it goes on cd for her? I usually like pooling out of ults but it's pointless if it doesn't put it on cd.