Exactly .... it's important with the vapor barrier to create a sealed envelope.If you used the plastic sheeting did you tape all the seams and seal anywhere it was cut such as for light fixtures?
Insulation is only one component since it provides a thermal resistance for heat gain/loss. Bat insulation will allow air to pass through.....the role of the vapor barrier is more of an air barrier.
It will stop warm air mixing with cold air which creates condensation. When properly installed and sealed the "vapor barrier" will also prevnt drafts which helps the comfort level.