  We consider particle acceleration in vacuum gaps in magnetospheres of black
holes powered through Blandford-Znajek mechanism and embedded into
radiatively-inefficient accretion flow (RIAF) environment. In such situation
the gap height is limited by the onset of gamma-gamma pair production on the
infrared photons originating from the RIAF. We numerically calculate
acceleration and propagation of charged particles taking into account the
detailed structure of electric and magnetic field in the gap and in the entire
black hole magnetosphere, radiative energy losses and interactions of gamma
rays produced by the propagated charged particles with the background radiation
field of RIAF. We show that the presence of the vacuum gap has clear
observational signatures. The spectra of emission from gaps embedded into a
relatively high luminosity RIAF are dominated by the inverse Compton emission
with a sharp, super-exponential cut-off in the very-high-energy gamma-ray band.
The cut-off energy is determined by the properties of the RIAF and is largely
independent of the structure of magnetosphere and geometry of the gap.The
spectra of the gap residing in low-luminosity RIAFs are dominated by
synchrotron / curvature emission with the spectra extending into 1-100 GeV
energy range. We also consider the effect of possible acceleration of protons
in the gap and find that proton energies could reach the ultra-high-energy
cosmic ray (UHECR) range only in extremely low luminosity RIAFs.
