  A quantum state is fully characterized by its density matrix or equivalently
by its quasiprobabilities in phase space. A scheme to identify the
quasiprobabilities of a quantum state is an important tool in the recent
development of quantum technologies. Based on our highly efficient vacuum
measurement scheme, we measure the quasiprobability $Q$-function of the
vibrational motion for a \Yb ion {\it resonantly} interacting with its internal
energy states. This interaction model is known as the Jaynes-Cummings model
which is one of the fundamental models in quantum electrodynamics. We apply the
capability of the vacuum measurement to study the Jaynes-Cummings dynamics,
where the Gaussian peak of the initial coherent state is known to bifurcate and
rotate around the origin of phase space. They merge at the so-called revival
time at the other side of phase space. The measured $Q$-function agrees with
the theoretical prediction. Moreover, we reconstruct the Wigner function by
deconvoluting the $Q$-function and observe the quantum interference in the
Wigner function at half of the revival time, where the vibrational state
becomes nearly disentangled from the internal energy states and forms a
superposition of two composite states. The scheme can be applied to other
physical setups including cavity or circuit-QED and optomechanical systems.
