  By using Nuclear Magnetic Resonance and ac-susceptibility, the characteristic
correlation times for the vortex dynamics, in an iron-based superconductor,
have been derived. Upon cooling, the vortex dynamics displays a crossover
consistent with a vortex glass transition. The correlation times, in the fast
motions regime, merge onto a universal curve which is fit by the Vogel-Fulcher
law, rather than by an Arrhenius law. Moreover, the pinning barrier shows a
weak dependence on the magnetic field which can be heuristically justified
within a fragile glass scenario. In addition, the glass freezing temperatures
obtained by the two techniques merge onto the de Almeida-Thouless line. Finally
the phase diagram for the mixed phase has been derived.
