Update-
As soon as I make vtime less than 25, the '?'s stop being detected. This sort of makes sense, since vtime = 25 means the program waits for 2.5 s per loop, 24 -> 2.4 s and so on.
But, in my limited experience, this sort of behaviour means that incoming bytes are discarded if there's nothing at that exact time to read them, which is very wierd.