Hi, It almost never rains the entire day in Vallarta, even during the rainy season. The weather in July, during the day is always very hot, humid and without any clouds! However around 7 or 8pm it will start raining very hard for a few hours. It rains 2 out of 3 evenings in general. This July we shot 6 weddings, and it rained for just 2 of them. So it could rain for your wedding, but that would be after the ceremony, during the reception, which a lot of hotel/villas can offer to have inside or you can rent a tent.
Pierre