That means: if you have vx_samples: 3and vtheta_samples: 20 the planner simulates trajectories for 3 different velocity inputs and 20 different angulat velocity inputs. So in the end a trajectory of one of the discrete vx_samples is scored, i.e. /cmd_vel is set to one of the 3 distinct vx_samples.
What you can do is increase vx_samples to something like 10 ~ 20 to allow for a finer discretization of your velocity sample space, i.e. the base controller can choose between 10 ~ 20 different velocities.