Complex sentence: The metaphor implies that, like a physical vacuum, other forces will tend to "rush in" to fill the vacuum as soon as it is created, perhaps in the form of an armed militia, military coup, warlord or dictator.
Split into simpler sentences: The metaphor implies that, like a physical vacuum, other forces will tend to fill the vacuum as soon as it is created. The implications of a power vacuum are that a powerful individual or group will "rush in" to fill that vacuum, perhaps in the form of an armed militia, military coup or dictator.