  A detailed analysis of the radiative $p\bar p$ annihilation is made in the
framework of a two-step formalism, the $p\bar p$ annihilates into meson
channels containing a vector meson with a subsequent conversion into a photon
via the vector dominance model (VDM). Both steps are derived from the
underlying quark model. First, branching ratios for radiative protonium
annihilation are calculated and compared with data. Then, details of the
isospin interference are studied for different models of the initial protonium
state and also for different kinematical form factors. The isospin interference
is shown to be uniquely connected to the $p\bar p - n\bar n $ mixing in the
protonium state. Values of the interference terms directly deduced from data
are consistent with theoretical expectations, indicating a dominant $p\bar p$
component for the $^1S_0$ and a sizable $n\bar n$ component for the $^3S_1$
protonium state. The analysis is extended to the $p\bar p \to \gamma \Phi$
transition, where the large observed branching ratio remains unexplained in the
VDM approach.
