How do the Voyager Spacecrafts avoid crashing into objects in interstellar space?
They don't. The Voyagers have no way to detect objects in the vicinity. We aren't using telescopes to look for objects in their paths either. We can get away with this because space is empty. Even in the densest region of the solar system (the asteroid belts), the average distance between objects is on the order of 1 million km.
When the Voyager program just started, there was some doubt if we'd be able to fly through the asteroid belt without hitting anything. Pioneer 10 and 11 were built in part to see if we could.