Was thinking that too at 1st, but only the Vamp got restrictions. He can go out and search for souls when that time comes. But he got lots of souls to live more centuries :3
If you think about it the demon also consumes souls and than the vampite sucks the blood containing those souls maybe it's like a loop?
Oh~ I didn't think of it that way. That would be more likely ヾ(☆▽☆) Great conclusion you got there!
Ohhhh. I see. That would be nice. (づ￣ ³￣)づ
What if the souls in the vampire runs out? ╥﹏╥