Depending on whether the vampire feeds on 3 people after an hour, or on 1 person every 20 minutes, I agree with the 256 and 4096 answers.
The fact that it's noon doesn't mean anything. It was never stated that the vampires in this particular riddle can't stand sunlight, and besides, they could get by in the subway system alone...
As for the compass lock thingie. I think the lock combination is NSWE (in that order).
In the grid, if you start at the A and take the north arrow, then - at the box you just arrived at - the south arrow, then the west arrow and then the east arrow, you arrive at the R.
Also: A + North + South + West + East + R = A+N+S+W+E+R = ANSWER.