Here's how the Vallarta Palace group booking worked for me:
We purchased one room in full (our room), which allowed our guests to not have to pay until 2 months prior to our wedding. By paying early for our room, it held our block of 25 rooms at no additional charge.
Then each guest booked their room through VP guest reservations directly (through our TA), so that it could count toward our total number of rooms (and we can get the promotional deals). Hope that helps!