The problem you describe is one that plagues even professional pizza operators, especially those who use deck ovens, which we try to simulate at home when we use pizza stones in our home ovens. The problem is the lack of sufficient movement of top heat to quickly dry the veggies as the pizza bakes. Pizza operators who have air impingement ovens have greater success with veggies on their pizzas because of the strong flow of air over the top of the pizzas to help evaporate some of the moisture contributed by the veggies, which they place on top of the cheese. But even they can have problems with moisture from veggies if there are too many of them. They can usually resolve the problem by special tricks they use involving screens, or else they use a lower bake temperature and a longer bake time to allow the pizza to bake longer and get rid of more of the moisture. It is also possible to put a coating of oil down on the pizza skin before dressing to serve as a barrier against the moisture seeping into the dough. A simple solution that I have seen Dom Demarco use at DiFara's is to add the veggies, or at least some of them, toward the end of the bake, just enough to heat them up without contributing too much additional moisture. That doesn't always work, as I can personally testify after having one of his veggie-laden pizzas.
There are also ways of preparing the veggies in advance to mitigate or minimize the "swamp" pizza phenomenon. You can try roasting the veggies, such as peppers, mushrooms and onions, and then coating them with olive oil to seal the outer surfaces. Or you can microwave them and blot as much moisture as possible with paper towels, following which you can lightly coat them with oil if you'd like.
If you are thinking in terms of a commercial oven, one type to avoid if you plan on using a lot of vegetables is the infrared ovens. They are known as being poor at drying veggies on the pizzas, and operators who own such ovens often use the paper blotting method to reduce liquids on the pizza.