  The vibrational dynamics of a model polymer glass is studied by Molecular
Dynamics simulations. The focus is on the "soft" monomers with high
participation to the lower-frequency vibrational modes contributing to the
thermodynamic anomalies of glasses. To better evidence their role, the
threshold to qualify monomers as soft is made severe, allowing for the use of
systems with limited size. A marked tendency of soft monomers to form
quasi-local clusters involving up to 15 monomers is evidenced. Each chain
contributes to a cluster up to about three monomers and a single cluster
involves monomer belonging to about 2-3 chains. Clusters with monomers
belonging to a single chain are rare. The open and tenuous character of the
clusters is revealed by their fractal dimension $d_f < 2$. The inertia tensor
of the soft clusters evidences their strong anisotropy in shape and remarkable
linear correlation of the two largest eigenvalues. Owing to the limited size of
the system, finite-size effects, as well as dependence of the results on the
adopted polymer length, cannot be ruled out.
