  The insulator-to-metal transition (IMT) of the simple binary compound of
vanadium dioxide VO$_2$ at $\sim 340$ K has been puzzling since its discovery
more than five decades ago. A wide variety of photon and electron probes have
been applied in search of a satisfactory microscopic mechanistic explanation.
However, many of the conclusions drawn have implicitly assumed a {\em
homogeneous} material response. Here, we reveal inherently {\em inhomogeneous}
behavior in the study of the dynamics of individual VO$_2$ micro-crystals using
a combination of femtosecond pump-probe microscopy with nano-IR imaging. The
time scales of the photoinduced bandgap reorganization in the ultrafast IMT
vary from $\simeq 40 \pm 8$ fs, i.e., shorter than a suggested phonon
bottleneck, to $\sim 200\pm20$ fs, with an average value of $80 \pm 25$ fs,
similar to results from previous studies on polycrystalline thin films. The
variation is uncorrelated with crystal size, orientation, transition
temperature, and initial insulating phase. This together with details of the
nano-domain behavior during the thermally-induced IMT suggests a significant
sensitivity to local variations in, e.g., doping, defects, and strain of the
microcrystals. The combination of results points to an electronic mechanism
dominating the photoinduced IMT in VO$_2$, but also highlights the difficulty
of deducing mechanistic information where the intrinsic response in correlated
matter may not yet have been reached.
