  Several studies on the transition voltage ($V_t$) are based on calculations
of the tunneling current within the Simmons model. In this paper devoted to
vacuum nano-junctions, we compare the Simmons results for $V_t$ with those
obtained from the exact Schrodinger equation by exactly including the classical
image effects. The comparison reveals that the Simmons estimates for $V_t$ are
completely unacceptable for nanogap sizes ($d$) at which image effects are
important. The Simmons treatment drastically overestimates these effects,
because it misses the famous 1/2 factor related to the fact that the image
interaction energy is a self energy. The maximum of the Simmons curve $V_t$ vs.
$d$ turns out to be merely an artefact of an inappropriate approximation.
Unlike the Simmons approach, the "exact" WKB method yields results, which
qualitatively agree with the exact ones; quantitative differences are
important, demonstrating that the transmission prefactor has a significant
impact on $V_t$. Further, we show that a difference between the work functions
of the left and right electrodes, which gives rise to a Volta field, may be
important for the ubiquitous asymmetry of the measured $I$-$V$-characteristics
in general, and for the different $V_t$-values at positive and negative biases
reported in vacuum nanojunctions in particular. The weak dependence $V_t =
V_t(d)$ found experimentally in vacuum nanojunctions contrasts to the
pronounced dependence obtained by including the exact electrostatic image
contribution into the Schrodinger equation. This demonstrates that not only the
molecular transport, but also the transport through a vacuum nanogap represents
a nontrivial problem, which requires further refinements, e.g., a realistic
description of contacts' geometry, retardation effects due to the finite
tunneling time, local phonons, surface plasmons or electron image states.
