Planet Hollywood IS included in the meal plan. The voucher you received from Planet Hollywood is not associated with the meal plan in any way--its just a promotion by PH to get more business. It is only good for ONE choice of a burger platter, a ceasar salad, OR Pasta Pomodoro (only one per reservation, no matter how many are in your party.) It doesn't include a beverage or gratuity.
However, if you are planning to go there anyway, you could use one less meal credit on the dining plan by using the voucher assuming that someone in your family wants one of the above menu choices.
By the way, the merchandise voucher is for a key chain.