We discuss the interplay between the piece-line regular and vertex-angle
singular boundary effects, related to integrability and chaotic features in
rational polygonal billiards. The approach to controversial issue of regular
and irregular motion in polygons is taken within the alternative deterministic
and stochastic frameworks. The analysis is developed in terms of the
billiard-wall collision distribution and the particle survival probability,
simulated in closed and weakly open polygons, respectively. In the multi-vertex
polygons, the late-time wall-collision events result in the circular-like
regular periodic trajectories (sliding orbits), which, in the open billiard
case are likely transformed into the surviving collective excitations
(vortices). Having no topological analogy with the regular orbits in the
geometrically corresponding circular billiard, sliding orbits and vortices are
well distinguished in the weakly open polygons via the universal and
non-universal relaxation dynamics.