When you have an active cellular data symbol (3G), then that counts towards the data plan.
When you have the active wifi symbol then it doesn't count towards the data plan.
Keep in mind that wifi will go to sleep 15 minutes after the phone screen turns dark if you are not plugged into the charger. At that point, cellular data will become active again. If you want to prevent that, turn off the cellular data connection during those times.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.